[RISCV] Use the 'si' lib call for (double (fp_to_sint/uint i32 X)) when F extension is enabled.

D80526 added custom lowering to pick the si lib call on RV64, but this custom handling is only enabled when the F and D extension are both disabled. This prevents the si library call from being used for double when F is enabled but D is not.

This patch changes the behavior so we always enable the Custom hook on RV64 and decide in ReplaceNodeResults if we should emit a libcall based on whether the FP type should be softened or not.

Differential Revision: https://reviews.llvm.org/D90817