[RISCV] Add support for fixed vector vselect

This patch adds support for fixed-length vector vselect. It does so by
lowering them to a custom unmasked VSELECT_VL node with a vector length
operand.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D96768

GitOrigin-RevId: d81161646a3d9a57750383f8e8b6c0b9abc7624b
4 files changed