[spv-in] Convert conditional backedges to break if.

[eddyb]

• fixes [spv-in] Conditional loop backedge not lowered to Statement::Loop with break_if expression #1977
• fixes Naga does not support our "SPIR-V conditional backedges" lifting for loops. EmbarkStudios/spirt#9

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The only test I added was the reduced do-while example (which only passes with my changes), but also note that the more complex Rust-GPU+SPIR-T motivating example, involving a Rust for loop, works as well now:

Before this PR	After this PR
thread 'main' panicked at 'wgpu error: Validation Error Caused by: In Device::create_shader_module Shader validation error: ┌─ :1:1 │ 1 │ │ naga::Function [1] Function [1] '' is invalid The `break` is used outside of a `loop` or `switch` context

[codecov-commenter]

Codecov Report

Merging #2290 (2c825d9) into master (53d62b9) will decrease coverage by 0.02%.
The diff coverage is 63.63%.

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@@            Coverage Diff             @@
##           master    #2290      +/-   ##
==========================================
- Coverage   82.19%   82.17%   -0.02%     
==========================================
  Files          82       82              
  Lines       44267    44342      +75     
==========================================
+ Hits        36384    36437      +53     
- Misses       7883     7905      +22     

Impacted Files	Coverage Δ
src/front/spv/function.rs	67.88% <60.75%> (-1.17%)	⬇️
src/front/spv/mod.rs	53.43% <66.66%> (-0.01%)	⬇️
src/block.rs	87.00% <100.00%> (+8.42%)	⬆️

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[jimblandy]

Rather than handling this in BlockContext::lower, it seems like it would be cleaner to detect conditional backedges in Frontend::next_block's match arm for Op::BranchConditional.

Rather than pushing a BodyFragment::If onto the end of ctx.bodies[body_idx] which BlockContext::lower then has to analyze, next_block could notice that the block it's finishing is a continue target (by checking ctx.mergers), verify that the conditional branch's true_id and false_id each refer to the loop header or loop merge block, synthesize a negation expression as necessary, and then stash the proper condition expression handle somewhere. (Perhaps BodyFragment::Loop should have a break_if: Option<Handle<Expression>> field?)

I think this should be less circuitous - we'll be taking advantage of what SPIR-V promises us, rather than building weird Naga IR and then taking it apart again.

[jimblandy]

This looks good - thank you for the fix!

I worry that that Op::BranchConditional match arm is getting pretty long and difficult to follow. I wonder if it's not possible to organize it a bit. But, that doesn't have to be part of this PR.