[x86] Flesh out the custom ISel for RMW aritmetic ops with used flags to

cover the bitwise operators.

Nothing really exciting here, this just stamps out the rest of the core
operations that can RMW memory and set flags.

Still not implemented here: ADC, SBB. Those will require more
interesting logic to channel the flags *in*, and I'm not currently
planning to try to tackle that. It might be interesting for someone who
wants to improve our code generation for bignum implementations.

Differential Revision: https://reviews.llvm.org/D37141

llvm-svn: 312768