Factor out code into HandleVirtRegDef, for consistency with

Handle{Virt,Phys}Reg{Def,Use}. Remove a redundant check
for register zero, and redundant checks for isPhysicalRegister.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@56412 91177308-0d34-0410-b5e6-96231b3b80d8