[GlobalIsel] Add failure memory order to LegalityQuery (NFC) (#162284)

The `cmpxchg` instruction has two memory orders, one for success and one
for failure.
Prior to this patch `LegalityQuery` only exposed a single memory order,
that of the success case. This meant that it was not generally possible
to legalize `cmpxchg` instructions based on their memory orders.

Add a `FailureOrdering` field to `LegalityQuery::MemDesc`; it is only
set for `cmpxchg` instructions, otherwise it is `NotAtomic`. I didn't
rename `Ordering` to `SuccessOrdering` or similar to avoid breaking
changes for out of tree targets.
The new field does not increase `sizeof(MemDesc)`, it falls into
previous padding bits due to alignment, so I'd expect there to be no
performance impact for this change.

Verified no breakage via check-llvm in build with AMDGPU, AArch64, and X86 targets
enabled.

0 files changed, 0 insertions, 0 deletions