[Constants] Handle FNeg in getWithOperands.

[fhahn]

Summary

Currently ConstantExpr::getWithOperands does not handle FNeg and
subsequently treats FNeg as binary operator, leading to an assertion
failure or segmentation fault if built without assertions.

Originally I reproduced this with llvm-dis on a bitcode file, which I
unfortunately cannot share and also cannot really reduce.

But PR45426 describes the same issue and has a reproducer with Clang, so
I'll go with that.

clang/test/CodeGen/constantexpr-fneg.c

+// RUN: %clang_cc1 -emit-llvm-bc -disable-llvm-passes -o %t.bc %s
+// RUN: llvm-dis %t.bc -o - | FileCheck %s
+
+// Test case for PR45426. Make sure we do not crash while writing bitcode
+// containing a simplify-able fneg constant expression. Check that the created
+// bitcode file can be disassembled and has the constant expressions simplified.
+//
+// CHECK-LABEL define i32 @main()
+// CHECK:      entry:
+// CHECK-NEXT:   %retval = alloca i32
+// CHECK-NEXT:   store i32 0, i32* %retval
+// CHECK-NEXT:   [[LV:%.*]] = load float*, float** @c
+// CHECK-NEXT:   store float 1.000000e+00, float* [[LV]], align 4
+// CHECK-NEXT:   ret i32 -1
+
+int a[], b;
+float *c;
+int main() {
+  return -(*c = &b != a);
+}

llvm/lib/IR/Constants.cpp

return ConstantExpr::getInsertElement(Ops[0], Ops[1], Ops[2],
                                           OnlyIfReducedTy);
   case Instruction::ExtractElement:
     return ConstantExpr::getExtractElement(Ops[0], Ops[1], OnlyIfReducedTy);
   case Instruction::InsertValue:
     return ConstantExpr::getInsertValue(Ops[0], Ops[1], getIndices(),
                                         OnlyIfReducedTy);
   case Instruction::ExtractValue:
     return ConstantExpr::getExtractValue(Ops[0], getIndices(), OnlyIfReducedTy);
+  case Instruction::FNeg:
+    return ConstantExpr::getFNeg(Ops[0]);
   case Instruction::ShuffleVector:
     return ConstantExpr::getShuffleVector(Ops[0], Ops[1], getShuffleMask(),
                                           OnlyIfReducedTy);
   case Instruction::GetElementPtr: {
     auto *GEPO = cast<GEPOperator>(this);
     assert(SrcTy || (Ops[0]->getType() == getOperand(0)->getType()));
     return ConstantExpr::getGetElementPtr(
         SrcTy ? SrcTy : GEPO->getSourceElementType(), Ops[0], Ops.slice(1),