[RISCV] Update combineBinOp_VLToVWBinOp_VL to check the number users after we...

[RISCV] Update combineBinOp_VLToVWBinOp_VL to check the number users after we know what combine we're going to do.

If we're forming vwadd(u).wv we don't care if the already wide
source operand has additional users. It's not affected by the
transform.

Fixes #59345