[llvm] [DAG] Support saturated truncate (PR #99418)

[David Green via llvm-commits]

================
@@ -14915,6 +14920,181 @@ SDValue DAGCombiner::visitEXTEND_VECTOR_INREG(SDNode *N) {
   return SDValue();
 }
 
+SDValue DAGCombiner::visitTRUNCATE_USAT(SDNode *N) {
+  EVT VT = N->getValueType(0);
+  SDValue N0 = N->getOperand(0);
+
+  auto MatchFPTOINT = [&](SDValue Val) -> SDValue {
+    if (Val.getOpcode() == ISD::FP_TO_SINT ||
+        Val.getOpcode() == ISD::FP_TO_UINT)
+      return Val;
+    return SDValue();
+  };
+
+  SDValue FPInstr;
+  if (N0.getOpcode() == ISD::SMAX) {
+    FPInstr = MatchFPTOINT(N0.getOperand(0));
+    if (!FPInstr)
+      FPInstr = MatchFPTOINT(N0.getOperand(1));
+  } else
+    FPInstr = MatchFPTOINT(N0);
+
+  if (FPInstr) {
----------------
davemgreen wrote:

It is common in LLVM to do
```
if (!FPInstr)
  return SDValue();
EVT FPVT = ...
```
It keeps the indentation down.

https://github.com/llvm/llvm-project/pull/99418