[MLIR] LLVM Dialect: add llvm.cmpxchg and improve llvm.atomicrmw custom parser

Summary:
Add a `llvm.cmpxchg` op as a counterpart to LLVM IR's `cmpxchg` instruction.
Note that the `weak`, `volatile`, and `syncscope` attributes are not yet supported.

This will be useful for upcoming parallel versions of affine.for and generally
for reduction-like semantics (especially for reductions that can't make use
of `atomicrmw`, e.g. `fmax`).

Reviewers: ftynse, nicolasvasilache

Reviewed By: ftynse

Subscribers: merge_guards_bot, jfb, mehdi_amini, rriddle, jpienaar, burmako, shauheen, antiagainst, arpith-jacob, mgester, lucyrfox, liufengdb, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D72995