[RISCV] Don't run combineBinOp_VLToVWBinOp_VL until after legalize types. NFCI (#84125)

I noticed this from a discrepancy in fillUpExtensionSupport between how
we apparently need to check for legal types for ISD::{ZERO,SIGN}_EXTEND,
but we don't need to for RISCVISD::V{Z,S}EXT_VL.

Prior to #72340, combineBinOp_VLToVWBinOp_VL only ran after type
legalization because it only operated on _VL nodes. _VL nodes are only
emitted during op legalization, which takes place **after** type
legalization, which is presumably why the existing code didn't need to
check for legal types.

After #72340 we now handle generic ops like ISD::ADD that exist before
op legalization and thus **before** type legalization. This meant that
we needed to add extra checks that the narrow type was legal in #76785.

I think the easiest thing to do here is to just maintain the invariant
that the types are legal and only run the combine after type
legalization.

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