Changes

[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