[AMDGPU] Fix adding modifiers when creating v_cmpx instructions.

Revision https://reviews.llvm.org/D122332 added a pattern transformation
where v_cmpx instructions are introduced. However, the modifiers are
not correctly inherited from the original operands. The patch
adds the source modifiers, if they are exist, or sets them to 0.

Reviewed By: foad

Differential Revision: https://reviews.llvm.org/D122489