After successfully commuting an instruction to be compatible with the
current VGPR MSB mode, update CurrentMode with the commuted
instruction's mode requirements. This locks in the mode bits the
commuted instruction relies on, preventing later instructions from
piggybacking and corrupting those bits.
Without this fix, a subsequent instruction needing a different mode
could piggyback onto the preceding s_set_vgpr_msb and change mode bits
that the commuted instruction depends on. For example, a nullopt src1
position (treated as 0) could be overwritten to a different value,
causing incorrect register encoding for the commuted instruction.
The fix still allows compatible piggybacking - instructions that only
add new mode bits without changing existing ones can still piggyback.
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