[X86] Crash when building SingleSource/UnitTests/Matrix/AMXINT8/t_gemm_bf16.cpp: Assertion `RegNo && RegNo < 388 && "Invalid register number!"'

[fhahn]

Building the tests in SingleSource/UnitTests/Matrix/AMXINT8/ added in afc7e6588b3b43351e49f697fe48283ec5056c11 crashes clang with a skylake-avx512 CPU.

I think this has been happening for a while.

Reduced IR reproducer:
https://llvm.godbolt.org/z/ax36ojxj5

[llvmbot]

@llvm/issue-subscribers-backend-x86

Author: Florian Hahn (fhahn)

Building the tests in `SingleSource/UnitTests/Matrix/AMXINT8/` added in [afc7e6588b3b43351e49f697fe48283ec5056c11](https://github.com/llvm/llvm-test-suite/commit/afc7e6588b3b43351e49f697fe48283ec5056c11) crashes `clang` with a `skylake-avx512` CPU.

I think this has been happening for a while.

Reduced IR reproducer:
https://llvm.godbolt.org/z/ax36ojxj5

[fhahn]

cc @LuoYuanke @XinWang10 @yubingex007-a11y