Update ABISysV_x86_64.cpp to use more efficient register finding calls.

Instead of looking up registers by name, we use the generic ID when we can.

Also added code that creates an extra frame when running expressions by pushing the current PC and FP and then hooking up the FP backchain. This code is "#if 0" out for now until we can pair it with unwinder fixes.

llvm-svn: 194035