[VPlan] Use correct non-FMF constructor in VPInstructionWithType createNaryOp (#137632)

Currently if we try to create a VPInstructionWithType without a FMF via
VPBuilder::createNaryOp we will use the constructor that asserts
`assert(isFPMathOp() && "this op can't take fast-math flags");`.

This fixes it by checking if FMFs have a value, similar to the other
createNaryOp overloads.

This is needed by #129508

Author: lukel97

llvm/lib/Transforms/Vectorize/LoopVectorizationPlanner.h

@@ -177,8 +177,11 @@ class VPBuilder {
                               Type *ResultTy,
                               std::optional<FastMathFlags> FMFs = {},
                               DebugLoc DL = {}, const Twine &Name = "") {
-    return tryInsertInstruction(new VPInstructionWithType(
-        Opcode, Operands, ResultTy, FMFs.value_or(FastMathFlags()), DL, Name));
+    if (FMFs)
+      return tryInsertInstruction(new VPInstructionWithType(
+          Opcode, Operands, ResultTy, *FMFs, DL, Name));
+    return tryInsertInstruction(
+        new VPInstructionWithType(Opcode, Operands, ResultTy, DL, Name));
   }
 
   VPInstruction *createOverflowingOp(unsigned Opcode,