Constructing Target triple with `ObjectFile::makeTriple` instead of just
with `Arch` and leaving the rest unknown. Also creating the subtarget
with the `CPU`. AMDGPU needs the full triple and `CPU` to disassemble
correctly.
To run a full test, also fixed a failure in `SIPreAllocateWWMRegs` with
the `$noreg` operand in `DBG_VALUE`.
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Co-authored-by: Matt Arsenault <arsenm2@gmail.com>
8710571aba