[RISCV] Fix missing def operand when creating VSETVLI pseudos

The register operand was not being marked as a def when it should be. No tests
for this in the main branch as there are not yet any pseudos without a
non-negative VLIndex.

Also change the type of a virtual register operand from unsigned to Register
and adjust formatting.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D92823