[RISCV][InsertVSETVLI] Default to MA not MU

This changes the default value used for mask policy from mask undisturbed to mask agnostic. In hardware, there may be a minor preference for ta/ma, but since this is only going to apply to instructions which don't use the mask policy bit, this is functionally mostly a nop. The main value is to make future changes to using MA when legal for masked instructions easier to review by reducing test churn.

The prior code was motivated by a desire to minimize state transitions between masked and unmasked code. This patch achieves the same effect using the demanded field logic (landed in afb45ffc), and there are no regressions I spotted in the test diffs. (Given the size, I have only been able to skim.) I do want to call out that regressions are possible here; the demanded analysis only works on a block local scope right now, so e.g. a tight loop mixing masked and unmasked computation might see an extra vsetvli or two.

Differential Revision: https://reviews.llvm.org/D133803