R600/SI: Fix using wrong operand indices when commuting

No test since the current SIISelLowering::legalizeOperands
effectively hides this, and the general uses seem to only fire
on SALU instructions which don't have modifiers between
the operands.

When trying to use legalizeOperands immediately after
instruction selection, it now sees a lot more patterns
it did not see before which break on this.

llvm-svn: 218527