[RISCV] Don't modify the SEW immediate on the V extension pseudo instructions...

[RISCV] Don't modify the SEW immediate on the V extension pseudo instructions after inserting VSETVLI.

Previously we set the value to -1, but the SEW information could
be useful for scheduling.

Reviewed By: frasercrmck, rogfer01

Differential Revision: https://reviews.llvm.org/D98062