Flatten nested insertvalues for Intel's SPIR-V driver

[maleadt]

Intel's graphics compiler (IGC) miscompiles aggregates built by nested insertvalue chains. Its TypesLegalizationPass splits aggregate stores into per-field stores by looking up each field in the chain. That lookup gives up on an insertvalue with more indices than the field it's looking for, so the store of that field is silently dropped. Aggregate phis are hit as well, because IGC lowers them to stores into an alloca.

For example, storing FlagFirst(flag, (a, b)), with struct FlagFirst; valid::Bool; value::Tuple{Float32,Int32}; end, loses flag. That breaks e.g. findmin-style reductions over (Bool, value) tuples. See JuliaGPU/OpenCL.jl#502, JuliaGPU/oneAPI.jl#259, and intel/intel-graphics-compiler#378. The bug has been in IGC since 2018 and is still on its master branch.

This PR adds a driver field to SPIRVCompilerTarget, so that consumers can say which driver will consume the SPIR-V. It defaults to :generic. When the driver is :intel, finish_ir! flattens multi-index insertvalues into single-index extract/insert pairs, which IGC handles correctly. The pass has to run after optimization, because InstCombine folds the pairs back together.

oneAPI.jl carries its own copy of this pass (from JuliaGPU/oneAPI.jl#548). It could pass driver=:intel and drop that copy.

The change is non-breaking: driver is a keyword argument with a default, and nothing changes unless a caller passes :intel. The @kwdef-generated positional constructor does gain an argument, but I'm not aware of anyone using it.

Tested on an Iris Xe (NEO 26.18, IGC 2.34) with OpenCL.jl passing driver=:intel. Without this change, the issue's reproducer, a nested struct, a loop-carried aggregate and oneAPI.jl#259's mapreduce all return wrong results. With it, they are all correct.

Fixes JuliaGPU/OpenCL.jl#502

[codecov]

Codecov Report

❌ Patch coverage is 96.55172% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 86.46%. Comparing base (8496909) to head (1fe0ea9).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/spirv.jl	96.55%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #953      +/-   ##
==========================================
+ Coverage   86.41%   86.46%   +0.05%     
==========================================
  Files          29       29              
  Lines        5674     5703      +29     
==========================================
+ Hits         4903     4931      +28     
- Misses        771      772       +1     

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