Track single def locals in importer

[AndyAyersMS]

Two separate commits (currently).

First just refactors existing use of lvSingleDef and is no diff.

Second repurposes lvSingleDef early to keep track of locals that are amenable to type updates, and then adds some type propagation to the late devirtualization callback.

[AndyAyersMS]

@briansull PTAL
cc @dotnet/jit-contrib

Diffs from second commit, mainly more devirtualization:

PMI Diffs for System.Private.CoreLib.dll, framework assemblies for x64 default jit
Summary:
(Lower is better)
Total bytes of diff: -14278 (-0.04% of base)
    diff is an improvement.
Top file regressions by size (bytes):
          13 : Microsoft.CSharp.dasm (0.00% of base)
          10 : System.Collections.Immutable.dasm (0.00% of base)
           8 : Microsoft.DotNet.InternalAbstractions.dasm (0.16% of base)
           2 : NuGet.Packaging.Core.dasm (0.02% of base)
Top file improvements by size (bytes):
       -3224 : System.Linq.Parallel.dasm (-0.29% of base)
       -2733 : System.Private.Xml.dasm (-0.08% of base)
       -1461 : System.Data.Common.dasm (-0.11% of base)
        -912 : Microsoft.DotNet.ProjectModel.dasm (-0.43% of base)
        -813 : Microsoft.Diagnostics.Tracing.TraceEvent.dasm (-0.03% of base)
60 total files with size differences (56 improved, 4 regressed), 69 unchanged.
Top method regressions by size (bytes):
         135 ( 8.27% of base) : Microsoft.CodeAnalysis.VisualBasic.dasm - VisualBasicCompilation:GetExtensionAttributeConstructor(byref):ref:this
          28 ( 1.29% of base) : Microsoft.CodeAnalysis.VisualBasic.dasm - ExpressionLambdaRewriter:VisitLambdaInternal(ref,ref):ref:this
          26 ( 4.66% of base) : Microsoft.CodeAnalysis.dasm - SuppressMessageAttributeState:DecodeLocalSuppressMessageAttributes(ref,ref):ref
          14 ( 1.58% of base) : System.Threading.Tasks.Dataflow.dasm - <>c:<.ctor>b__9_6(ref,ref):this (10 methods)
          13 ( 0.66% of base) : Microsoft.CSharp.dasm - SymbolTable:SetParameterAttributes(ref,ref,int)
Top method improvements by size (bytes):
        -288 (-12.20% of base) : Microsoft.DotNet.ProjectModel.dasm - LockFileReader:ReadObject(ref,ref):ref (5 methods)
        -264 (-11.67% of base) : NuGet.ProjectModel.dasm - LockFileFormat:ReadObject(ref,ref):ref (5 methods)
        -260 (-19.05% of base) : System.Linq.Parallel.dasm - CountAggregationOperator`1:InternalAggregate(byref):int:this (5 methods)
        -260 (-18.91% of base) : System.Linq.Parallel.dasm - LongCountAggregationOperator`1:InternalAggregate(byref):long:this (5 methods)
        -260 (-16.83% of base) : System.Linq.Parallel.dasm - AnyAllSearchOperator`1:Aggregate():bool:this (5 methods)
Top method regressions by size (percentage):
         135 ( 8.27% of base) : Microsoft.CodeAnalysis.VisualBasic.dasm - VisualBasicCompilation:GetExtensionAttributeConstructor(byref):ref:this
           3 ( 6.00% of base) : Microsoft.Diagnostics.Tracing.TraceEvent.dasm - CtfString:Read(ref):ref:this
           8 ( 5.93% of base) : Microsoft.DotNet.InternalAbstractions.dasm - FileWrapper:CreateEmptyFile(ref):this
          13 ( 5.70% of base) : System.Collections.Immutable.dasm - ImmutableArray`1:System.Collections.IStructuralEquatable.GetHashCode(ref):int:this (5 methods)
           2 ( 5.13% of base) : Microsoft.CodeAnalysis.CSharp.dasm - ReferenceManager:WeakIdentityPropertiesEquivalent(ref,ref):bool:this
Top method improvements by size (percentage):
         -43 (-25.15% of base) : System.Data.Common.dasm - ADP:IsCatchableOrSecurityExceptionType(ref):bool
         -54 (-25.12% of base) : System.Data.Common.dasm - ADP:IsCatchableExceptionType(ref):bool
         -52 (-21.94% of base) : Microsoft.CodeAnalysis.dasm - ExceptionHandlerContainerScope:FreeBasicBlocks():this
         -35 (-19.55% of base) : System.Net.Primitives.dasm - Marvin:GenerateSeed():long
         -35 (-19.55% of base) : System.Private.Xml.dasm - Marvin:GenerateSeed():long
474 total methods with size differences (428 improved, 46 regressed), 190802 unchanged.

[briansull]

Do we still have varDsc->lvSingleDef here?
It is important to mark the 'this' pointer as a single def variable.

[AndyAyersMS]

it is set later on (as you note below).

[briansull]

Is this where we get the 'this' pointers lcSingleDef set to true?

[AndyAyersMS]

Yes.

[briansull]

You may be able to close Issue #17949 after this change.

[jkotas]

Computing class attribs is relatively expensive operation. It computes a ton more than just the CORINFO_FLG_SHAREDINST flag that you care about here. It would be better to change mergeClasses to return NULL when any of the types is shared.

[AndyAyersMS]

Sure. These type updates shouldn't happen all that often but moving the check should be simpler and faster.