[riscv] Prefer to use previous VL for scalar move instructionsK

[riscv] Prefer to use previous VL for scalar move instructionsK

This patch is an alternative to a piece of D125270. Its direct motivation is to fix a wrong code bug (described below), but somewhat unexpectedly, it also results in a significant code quality improvement for idiomatic fixed length vector patterns.

The existing transform is simply wrong in its current location. We are correct about the fact that the scalar move itself can use the previous vsetvli, but we loose track of the fact that later instructions might depend on the state change represented. That is, the actual value of VL in the register is different than the abstract state thinks it is. Not simply due to precision of modeling, but e.g. the VL register could contain 3 when the abstract state says it is 1. This is annoying hard to demonstrate in practice due to differences in policy flags on the intrinsics, but this is at least a latent wrong code bug.

The code quality benefit comes from the fact we don't need to tie this to explicit vsetvli instructions at all. We can propagate the abstract state, and reduce a) the number of transitions, or b) the cost of those transitions. It turns out we have a bunch of cases - in tests at least - where fixed length AVLs are known non-zero, and we can leave VL unchanged while changing VTYPE.

Differential Revision: https://reviews.llvm.org/D125337