[AMDGPU] Fix for folding v2.16 literals.
It was found some packed immediate operands (e.g. <half 1.0, half 2.0>) are
incorrectly processed so one of two packed values were lost.
Introduced new function to check immediate 32-bit operand can be folded.
Converted condition about current op_sel flags value to fall-through.
Reviewed By: rampitec
Differential Revision: https://reviews.llvm.org/D87158