[RISCV] Use vmv.v.i for insertion into lane 0 of undef vector when profitable

If we're initializing lane 0 of an undef vector, we can optionally write to other lanes of the vector. Doing so may require additional work, so we don't want to e.g. always use a splat. However, since we don't have an immediate form of vmv.s.x it's useful to use a vmv.v.i if the work required is expected to be equal in practice.  We restrict this to when LMUL <= 1 to a) prevent doing additional work at higher LMULs, and b) avoid overconstraining the register allocator.

At the moment, the new utility is only used by one case in INSERT_VECTOR_ELT lowering. My expectation is that we will reuse this in a couple other places, but each of those deserve individual review.

This change is inspired by D137530, but is not directly related to it. I vaguely remember we discussed the tradeoffs of using vmv.v.i in another recent review, but couldn't find it.

Differential Revision: https://reviews.llvm.org/D139648