Polyfill this for HLSL using an atomic add with the operand negated.
Fixed: tint:1130
Change-Id: Ifa32d58973f1b48593ec0f6320f47f4358a5a3a9
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/62760
Auto-Submit: James Price <jrprice@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
James Price