[RISCV][InsertVSETVLI] Handle partially transparent instructions in PRE

The PRE implementation was being overly strict when checking to see if a vsetvli was removed in the current block. For instructions which don't use all the fields of VTYPE or VL, we can propagate a changed state past the first instruction with an SEW operand and remove a vsetvli later in the block. We do need to be careful now to ensure that the state convergences before the end of the block or we'd invalidate the cached data flow results.

Taking a step back, we're modeling the effect of the emitVSETVLIs pass which runs just after PRE. This is unfortunate, and makes me think we should probably reevaluate doing the PRE as a post-pass instead of as surgery in the data flow phases. Doing that requires us to get more aggressive about mutating user written vsetvlis which we've tried not to do up to now, but well, maybe it's time? Anyways, that's a thought for the future, not something I'm proposing doing now.

Differential Revision: https://reviews.llvm.org/D142409