[SLP] createOp - fix null dereference warning. NFCI.

Only attempt to propagateIRFlags if we have both SelectInst - afaict we shouldn't have matched a min/max reduction without both SelectInst, but static analyzer doesn't know that.

GitOrigin-RevId: b49c41afbaa212cc15343af68c3293ab929a2d34
1 file changed