[X86] Simplify X86 epilogue frame size calculation, NFC

Sink the insertion of "pop ebp" out of the frame size calculation
branches. They all check for HasFP.

Our handling of CLEANUPRET and CATCHRET was equivalent, both are
funclets and use the same frame size. We can eliminate the CLEANUPRET
case.

Hoist the hasFP(MF) query into a local bool.

Rename TargetMBB to CatchRetTarget to be more descriptive.

Eliminate the Optional<unsigned> RetOpcode local, now that it has one
use.

It's only a net savings of 10 lines, but hopefully it's *slightly* more
readable.

llvm-svn: 315000