[AMDGPU] Remove isLiteralConstant and isLiteralConstantLike

isLiteralConstant and isLiteralConstantLike were similar to
!isInlineConstant with slight differences like handling isReg operands.

To avoid a profusion of similar functions with undocumented differences,
this patch removes all the isLiteralConstant* variants. Callers are responsible
for handling the isReg case.

Differential Revision: https://reviews.llvm.org/D125759