[llvm] [mlir] Add usub_cond and usub_sat operations to atomicrmw (PR #105568)

[Matt Arsenault via llvm-commits]

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@@ -751,8 +751,16 @@ class AtomicRMWInst : public Instruction {
     /// *p = ((old == 0) || (old u> v)) ? v : (old - 1)
     UDecWrap,
 
+    /// Subtract only if result would be positive.
+    /// *p = (old u>= v) ? old - v : old
+    USubCond,
+
+    /// Subtract with clamping of negative results to zero.
+    /// *p = (old u>= v) ? old - v : 0
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arsenm wrote:

Comment in terms of the intrinsic? Also could update the phrasing to match langref 

https://github.com/llvm/llvm-project/pull/105568