R600/SI: Partially move operand legalization to post-isel hook.

R600/SI: Partially move operand legalization to post-isel hook.

Disable the SGPR usage restriction parts of the DAG legalizeOperands.
It now should only be doing immediate folding until it can be replaced
later. The real legalization work is now done by the other
SIInstrInfo::legalizeOperands

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218531 91177308-0d34-0410-b5e6-96231b3b80d8