[X86] Make ADD*_DB post-RA pseudos and expand them in expandPostRAPseudo.

[X86] Make ADD*_DB post-RA pseudos and expand them in expandPostRAPseudo.

These are used to help convert OR->LEA when needed to avoid avoid a copy. They
aren't need after register allocation.

Happens to remove an ugly goto from X86MCCodeEmitter.cpp

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@356356 91177308-0d34-0410-b5e6-96231b3b80d8