[LegalizeVectorOps] Split most of ExpandStrictFPOp into a separate...

[LegalizeVectorOps] Split most of ExpandStrictFPOp into a separate UnrollStrictFPOp method. Call that method from ExpandUINT_TO_FLOAT.

ExpandStrictFPOp calls ExpandUINT_TO_FLOAT. Previously, ExpandUINT_TO_FLOAT
returned SDValue() if it wasn't able to handle and needed to unroll.
Then ExpandStrictFPOp would detect his SDValue() and do the unroll.

After this change, ExpandUINT_TO_FLOAT will directly call
UnrollStrictFPOp and return the unrolled result.