[AMDGPU] Stop using getMinimalPhysRegClass in LowerFormalArguments

NFCI. The motivation for this is avoid problems in future if we add new
classes containing only a subset of all VGPRs, or a subset of all SGPRs.
getMinimalPhysRegClass would favour these smaller classes, which is not
what we want here.

Differential Revision: https://reviews.llvm.org/D121914