[RISCV] Add merge operands to more RISCVISD::*_VL opcodes.

This adds a merge operand to all of the binary _VL nodes. Including
integer and widening. They all share multiclasses in tablegen
so doing them all at once was easiest.

I plan to use FADD_VL in an upcoming patch. The rest are just for
consistency to keep tablegen working.

This does reduce the isel table size by about 25k so that's nice.

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D130816