[AMDGPU] Add verification for amdgcn.init.exec.from.input (#128172)
Check that the input register is an inreg argument to the parent
function. (See the comment in `IntrinsicsAMDGPU.td`.)
This LLVM defect was identified via the AMD Fuzzing project.
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Co-authored-by: Matt Arsenault <arsenm2@gmail.com>