[RISCV] Branchless lowering for (select (x < 0), TrueConstant, FalseConstant)...

[RISCV] Branchless lowering for (select (x < 0), TrueConstant, FalseConstant) and (select (x >= 0), TrueConstant, FalseConstant)

This patch reduces the number of unpredictable branches

(select (x < 0), y, z)  -> x >> (XLEN - 1) & (y - z) + z
(select (x >= 0), y, z) -> x >> (XLEN - 1) & (z - y) + y

Reviewed By: craig.topper, reames

Differential Revision: https://reviews.llvm.org/D137949