Spiller may unfold load / mod / store instructions as an optimization when the would...

Spiller may unfold load / mod / store instructions as an optimization when the would be loaded value is available in a register. It needs to check if it's legal to clobber the register. Also, the register can contain values of multiple spill slots, make sure to check all instead of just the one being unfolded.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@67068 91177308-0d34-0410-b5e6-96231b3b80d8