[AMDGPU] Allow accvgpr_read/write decode with opsel

These two instructions are VOP3P and have op_sel_hi bits,
however do not use op_sel_hi. That is recommended to set
unused op_sel_hi bits to 1. However, we cannot decode
both representations with 1 and 0 if bits are set to
default value 1. If bits are set to be ignored with '?'
initializer then encoding defaults them to 0.

The patch is a hack to force ignored '?' bits to 1 on
encoding for these instructions.

There is still canonicalization happens on disasm print
if incoming values are non-default, so that disasm output
does not match binary input, but this is pre-existing
problem for all instructions with '?' bits.

Fixes: SWDEV-272540

Differential Revision: https://reviews.llvm.org/D96543