[RISCV] Optimize lowering of floating-point -0.0

This idea has come up in several reviews -- D115978 and D105902 -- so I
can't take any credit for the idea. Instead of using a constant pool to
lower -0.0, we can emit a sequence of two instructions:

    fmv.[hwd].x freg, zero
    fsgnjn.[hsd] freg, freg, freg

This is only done when the floating-point type is legal.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D117687