physical register as candidate for common subexpression elimination
in MachineCSE.
This fixes a bug on PowerPC in MultiSource/Applications/oggenc/oggenc
caused by MachineCSE invalidly merging two separate DYNALLOC insns.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@167855 91177308-0d34-0410-b5e6-96231b3b80d8