[RISCV] Hoist VSETVLI vlmax, vtype out of scalable loops

[RISCV] Hoist VSETVLI vlmax, vtype out of scalable loops

This is a straight forward extension of the PRE transform introduced in D124869 to handle the VLMAX case.

The test changes here look quite positive. This surprised me until I realized that all the tests are using @llvm.vscale to figure out the VLMAX, not the llvm.riscv.vsetvlmax intrinsic. If they'd used the later, these would have been full redundancy cases and fully handled by the data flow. I'm not really sure if use of vscale here is representative or not. If it is, we should probably look at using VSETVLI to lower vscale rather than a raw read of vlenb and some math.

Differential Revision: https://reviews.llvm.org/D126338