[RISCV] Add support for selecting vid.v from build_vector

This patch optimizes a build_vector "index sequence" and lowers it to
the existing custom RISCVISD::VID node. This pattern is common in
autovectorized code.

The custom node was updated to allow it to be used by both scalable and
fixed-length vectors, thus avoiding pattern duplication.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D96332