[llvm] [RISCV] Lower SELECT's with one constant more efficiently using Zicond (PR #143581)

[Ryan Buchner via llvm-commits]

bababuck wrote:

Updated for the above suggestions.

Slight refactoring to address the "It's bad practice to create a SDNode that isn't guaranteed to be used."

https://github.com/llvm/llvm-project/pull/143581