[AMDGPU] Fix implicit $vcc operands after parsing MIR (#87781)

MIParser checks that implicit operands match the instruction definition,
so they have to be $vcc even in wave32 mode. Use the mirFileLoaded hook
to fix them after MIParser's checks, converting them to $vcc_lo which is
what that rest of CodeGen expects.

This is all just extending the fixImplicitOperands hack which was
introduced with GFX10, but at least it makes it possible to write a MIR
test which creates the same instructions that normal CodeGen would
generate.