[RISCV][InsertVSETVLI] Treat vmv.v.i as-if it were vmv.s.x when VL=1, and inactive...

[RISCV][InsertVSETVLI] Treat vmv.v.i as-if it were vmv.s.x when VL=1, and inactive lanes are undefined

A vmv.v.i/x splats the immediate to all active lanes. For the active lanes, this is the same as vmv.s.x which inserts one scalar into the low lane. If we can ignore all the inactive lanes (because they are known undefined), then the two are semantically equivalent. We already reason about compatible VL/VTYPE combinations for vmv.s.x, apply the same logic to vmv.v.i.

Unlike a vmv.s.x, we do need to be careful not to increase LMUL. A splat instruction is probably linear in LMUL, so restrict this to LMUL1.

Differential Revision: https://reviews.llvm.org/D152845