[AMDGPU][NFC] Remove redundant condition

Condition `LiteralCount` is checked both in an outer and in an inner
`if` statement in `SIInstrInfo::verifyInstruction()`. This patch removes
the redundant inner check.

The issue was found using `clang-tidy` check under review
`misc-redundant-condition`. See https://reviews.llvm.org/D81272.

Differential Revision: https://reviews.llvm.org/D82555