Verify IR after our own validation.

[maleadt]

Another hotfix for #510: Turns out the LLVM verifier can be unhappy about non-void kernels:

julia> invalid_kernel() = throw()
       @oneapi invalid_kernel()
ERROR: LLVM error: Calling convention requires void return type
{}* ()* @_Z14invalid_kernel

Stacktrace:
  [1] verify(mod::LLVM.Module)
    @ LLVM ~/Julia/pkg/LLVM/src/analysis.jl:10

... so make sure we validated the IR ourselves first.

It's not perfect, as our reflection macros invoke the driver with validate=false:

julia> @device_code_llvm @oneapi invalid_kernel()
; GPUCompiler.CompilerJob{GPUCompiler.SPIRVCompilerTarget, oneAPI.oneAPICompilerParams}(MethodInstance for invalid_kernel(), CompilerConfig for GPUCompiler.SPIRVCompilerTarget, 0x0000000000007f13)
ERROR: LLVM error: Calling convention requires void return type
{}* ()* @_Z14invalid_kernel

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +8.46% 🎉

Comparison is base (c916b4e) 74.35% compared to head (605302e) 82.82%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #514      +/-   ##
==========================================
+ Coverage   74.35%   82.82%   +8.46%     
==========================================
  Files          24       24              
  Lines        3276     3289      +13     
==========================================
+ Hits         2436     2724     +288     
+ Misses        840      565     -275     

Files Changed	Coverage Δ
src/driver.jl	88.66% <100.00%> (+26.60%)	⬆️

... and 10 files with indirect coverage changes

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