[AMDGPU][AsmParser][NFC] Clean up the implementation of KImmFP operands.

addKImmFPOperands() duplicates the KImmFP-specific logic implemented in
addLiteralImmOperand() and therefore can be removed.

Part of <https://github.com/llvm/llvm-project/issues/62629>.

Reviewed By: foad

Differential Revision: https://reviews.llvm.org/D154427