Jit: fix a few issues with single def local tracking

[AndyAyersMS]

Fixes for three issues that came up in desktop testing.

1. Don't track class types for locals when in import only mode,
   since the jit may be looking at an uninstantiated generic method.
   In these cases the jit's use of TYP_REF to represent CORINFO_TYPE_VAR
   confuses the tracking code. In import only mode the jit is not going
   to use the information, so there is no need to track it at all.

   Import only mode is tied to verification and CoreCLR runs in full
   trust mode, so no test was added.

2. Allow lvaUpdateClass to be called more than once but assert
   that the second and subsequent calls provide the exact same update
   as the first call.

   This can happen for single def ref class locals whose definition
   block is reimported. Reimportation is triggered by some tolerable
   non-ref type mismatches at block joins and so ref type information
   should be consistent for each importation attempt.

   Add a couple of IL test cases translated from destkop MC++ tests
   that will trigger this kind of reimportation.

3. Bail out of devirtualization (for desktop only) if the base class
   has precise initialization semantics, since desktop seems to trigger
   class initialization for ref type callvirts (contrary to ECMA355 I.8.9.5).
   See #4853 for some discussion. Since this is desktop only behavior
   no test was added.

[AndyAyersMS]

@briansull PTAL
cc @dotnet/jit-contrib

No diffs in CoreCLR on jit-diff over frameworks.

[briansull]

Looks Good

[AndyAyersMS]

Looks like the new tests fail on x86. Not too surprising since I ported them from x64 MC++.

Let me see if I can fix them to be less arch-dependent.

[briansull]

One additional comment on this

Reimportation is triggered by some tolerable non-ref type mismatches at block joins and so ref type information should be consistent for each importation attempt.

I think that re-importation is also triggered when a null is push on the IL stack on one path and then on the alternate path a non-null ref type is pushed.

[AndyAyersMS]

The cases I see in the importer that trigger reimportation do not involve ref types:

• int32 and native int --> native int
• double and float --> double
• & and native int --> &
• & and int32 --> &