[RISCV] Fold vector binary operatrion into select with identity constant.

[RISCV] Fold vector binary operatrion into select with identity constant.

This patch implements shouldFoldSelectWithIdentityConstant for RISCV. It would try to generate vmerge after the binary instruction and let them folded to maksed instruction later.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D131551