[InstCombine] Check FPMathOperator for Ctx before FMF check

We need to check FPMathOperator for Ctx instruction before checking fast
math flag on this Ctx.

Ctx is not always an FPMathOperator, so explicitly check for it.

Fixes #71548.

Author: annamthomas

llvm/lib/Analysis/InstructionSimplify.cpp

@@ -6606,7 +6606,8 @@ static Value *simplifyBinaryIntrinsic(Function *F, Value *Op0, Value *Op1,
     // float, if the ninf flag is set.
     const APFloat *C;
     if (match(Op1, m_APFloat(C)) &&
-        (C->isInfinity() || (Q.CxtI->hasNoInfs() && C->isLargest()))) {
+        (C->isInfinity() || (isa<FPMathOperator>(Q.CxtI) &&
+                             Q.CxtI->hasNoInfs() && C->isLargest()))) {
       // minnum(X, -inf) -> -inf
       // maxnum(X, +inf) -> +inf
       // minimum(X, -inf) -> -inf if nnan

llvm/test/Transforms/InstCombine/minimum.ll

@@ -471,3 +471,29 @@ define double @negated_op_extra_use(double %x) {
   %r = call double @llvm.minimum.f64(double %negx, double %x)
   ret double %r
 }
+
+; Testcase from PR 71548.
+define void @pr71548() {
+; CHECK-LABEL: @pr71548(
+; CHECK-NEXT:    [[C0:%.*]] = load atomic double, ptr addrspace(1) null unordered, align 8
+; CHECK-NEXT:    [[C1:%.*]] = load atomic i32, ptr addrspace(1) null unordered, align 4
+; CHECK-NEXT:    [[C2:%.*]] = sitofp i32 [[C1]] to double
+; CHECK-NEXT:    [[CRES_I:%.*]] = call noundef double @llvm.minimum.f64(double [[C0]], double [[C2]])
+; CHECK-NEXT:    [[C3:%.*]] = fcmp ult double [[CRES_I]], 0.000000e+00
+; CHECK-NEXT:    [[C_NOT16:%.*]] = icmp eq i32 [[C1]], 0
+; CHECK-NEXT:    [[COR_COND45:%.*]] = or i1 [[C3]], [[C_NOT16]]
+; CHECK-NEXT:    call void @llvm.assume(i1 [[COR_COND45]])
+; CHECK-NEXT:    ret void
+;
+  %c0 = load atomic double, ptr addrspace(1) null unordered, align 8
+  %c1 = load atomic i32, ptr addrspace(1) null unordered, align 4
+  %c2 = sitofp i32 %c1 to double
+  %cres.i = call noundef double @llvm.minimum.f64(double %c0, double %c2)
+  %c3 = fcmp ult double %cres.i, 0.000000e+00
+  %c.not16 = icmp eq i32 %c1, 0
+  %cor.cond45 = or i1 %c3, %c.not16
+  call void @llvm.assume(i1 %cor.cond45)
+  ret void
+}
+
+declare void @llvm.assume(i1)