[AMDGPU] Make OMod explicit for V_CVT_{U,I}*

Make OMod explicit instead of implied by HasModifiers in the
operand list. Requires explicitly setting HasOMod=1 for
irregular OMod usage in instruction V_CVT_{U,I}*

Reviewed By: foad

Differential Revision: https://reviews.llvm.org/D97587

Change-Id: I230e1476f529e816eec60e242531f23a99e3839f