[RISCV] Add special case for (select cc, 1.0, 0.0) to lowerSELECT

Use sint_to_fp instead of select.
Reduce the number of branch instructions and
avoid generating TargetConstantPool for double.

(select cc, 1.0, 0.0) -> (sint_to_fp (zext cc))
https://alive2.llvm.org/ce/z/aoEcd9
https://godbolt.org/z/n543Y9v3e

(select cc, 0.0, 1.0) -> (sint_to_fp (zext (xor cc, 1)))
https://alive2.llvm.org/ce/z/zngvSB

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D151719