[llvm] [InstCombine] Fold fcmp ogt (x - y), 0 into fcmp ogt x, y #85245 (PR #85506)

[Matt Arsenault via llvm-commits]

================
@@ -8045,6 +8045,42 @@ Instruction *InstCombinerImpl::visitFCmpInst(FCmpInst &I) {
   Constant *RHSC;
   if (match(Op0, m_Instruction(LHSI)) && match(Op1, m_Constant(RHSC))) {
     switch (LHSI->getOpcode()) {
+    case Instruction::FSub:
+      switch (Pred) {
+      default:
+        break;
+      case FCmpInst::FCMP_UGT:
+      case FCmpInst::FCMP_ULT:
+      case FCmpInst::FCMP_UNE:
+      case FCmpInst::FCMP_OEQ:
+      case FCmpInst::FCMP_OGE:
+      case FCmpInst::FCMP_OLE: {
+        if (!LHSI->hasNoNaNs() && !LHSI->hasNoInfs() &&
+            !isKnownNeverInfinity(LHSI->getOperand(1), /*Depth=*/0,
+                                  getSimplifyQuery().getWithInstruction(&I)) &&
+            !isKnownNeverInfinity(LHSI->getOperand(0), /*Depth=*/0,
+                                  getSimplifyQuery().getWithInstruction(&I)))
+          break;
+      }
+        LLVM_FALLTHROUGH;
----------------
arsenm wrote:

I think we can just use [[fallthrough]] now. Also belongs before the } 

https://github.com/llvm/llvm-project/pull/85506