[RISCV] Legalize select when Zbt extension available

[RISCV] Legalize select when Zbt extension available

The custom expansion of select operations in the RISC-V backend
interferes with the matching of cmov instructions. Legalizing
select when the Zbt extension is available solves that problem.

Reviewed By: lenary, craig.topper

Differential Revision: https://reviews.llvm.org/D93767