Tell GPUCompiler which driver consumes the SPIR-V

[maleadt]

Fixes #502.

Intel's graphics compiler drops struct fields when storing aggregates built by nested insertvalues. For example, the Bool in FlagFirst(flag, (a, b)) is lost, and so is the flag in (Bool, value) reduction tuples. The cause is a bug in IGC's TypesLegalizationPass (intel/intel-graphics-compiler#378). Our SPIR-V is valid.

JuliaGPU/GPUCompiler.jl#953 works around this by flattening nested insertvalues when SPIRVCompilerTarget(; driver=:intel). This PR reports the driver (:intel for Intel GPUs only, since Intel's CPU runtime uses a different compiler; otherwise :pocl, :nvidia or :generic) and adds a regression test.

Requires GPUCompiler 2.9, which includes JuliaGPU/GPUCompiler.jl#953.

With it, the full test suite passes on an Iris Xe (NEO 26.18, IGC 2.34), including the new test.

[codecov]

Codecov Report

❌ Patch coverage is 44.44444% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.12%. Comparing base (57e2c81) to head (16abfe1).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
src/compiler/compilation.jl	44.44%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #504      +/-   ##
==========================================
- Coverage   83.34%   83.12%   -0.22%     
==========================================
  Files          19       19              
  Lines        1603     1612       +9     
==========================================
+ Hits         1336     1340       +4     
- Misses        267      272       +5     

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