[RISCV][WIP] Use vsetvli x0, x0 in more cases.

If the AVL is a virtual register defined by a vsetvli with the same
vlmax we need and the previous vsetvli we saw in the data flow also
has that vlmax, we can use the x0, x0 form when we insert a vsetvli.

Not only does this avoid an update of the VL physical register, but
it may allow doLocalPostpass to completely remove the inserted vsetvli
by rewriting the vtype of the previous vsetvli.

Differential Revision: https://reviews.llvm.org/D148735