[x86] Extend the manual ISel of `add` and `sub` with both RMW memory

operands and used flags to support matching immediate operands.

This is a bit trickier than register operands, and we still want to fall
back on a register operands even for things that appear to be
"immediates" when they won't actually select into the operation's
immediate operand. This also requires us to handle things like selecting
`sub` vs. `add` to minimize the number of bits needed to represent the
immediate, and picking the shortest immediate encoding. In order to
that, we in turn need to scan to make sure that CF isn't used as it will
get inverted.

The end result seems very nice though, and we're now generating
optimal instruction sequences for these patterns IMO.

A follow-up patch will further expand this to other operations with RMW
memory operands. But handing `add` and `sub` are useful starting points
to flesh out the machinery and make sure interesting and complex cases
can be handled.

Thanks to Craig Topper who provided a few fixes and improvements to this
patch in addition to the review!

Differential Revision: https://reviews.llvm.org/D37139

llvm-svn: 312764

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