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.
This is a purely stylistic comment - but I find the placement of merge between mask and VL slightly surprising. Would it be easy to make the order merge, mask, VL? This would keep this matching the MI operand order as much as possible while still obeying the commutative restriction to not have merge first in the sequence.