[VectorCombine] Don't use Builder.Create* methods when replacing flags

[RKSimon]

As detailed here: #148350 (comment)

Since we use IRBuilder in VectorCombine, the result may point to an existing cast instruction instead of CastOp (NewOp). Therefore, it may propagate flags into an incorrect instruction. I recommend using CastInst::Create instead of Builder.CreateCast.

Technically we only need to update the instructions where we're replacing the flags, but probably we want to be consistent at least per-fold if not everywhere.

CC @rhyadav @dtcxzyw

[rhyadav]

Thanks @RKSimon i would like to contribute to this, request you to assign