[x86] Teach the backend to fold more read-modify-write memory operands

to instructions.

These can't be reasonably matched in tablegen due to the handling of
flags, so we have to do this in C++ code. We only did it for `inc` and
`dec` historically, this starts fleshing that out to more interesting
instructions. Notably, this handles transfering operands to `add` and
`sub`.

Currently this forces them into a register. The next patch will add
support for keeping immediate operands as immediates. Then I'll extend
this beyond just `add` and `sub`.

I'm not super thrilled by the repeated switches in the code but
everything else I tried was really ugly or problematic.

Many thanks to Craig Topper for the suggestions about where to even
begin here and how to make this stuff work.

Differential Revision: https://reviews.llvm.org/D37130

llvm-svn: 311806