[RISCV] Make visibility of overridden methods in RISCVISelLowering match the parent

Currently, some fairly arbitrary subset of overriden methods in
RISCVISelLowering are private rather than public (which is the
visibility they have in TargetLowering). I suspect this is a holdover
from too closely copying another backend.

D78545 pointed out this can be difficult for some downstream patches,
and nobody has come forward to suggest a reason for keeping the
visibility as-is.

This commit simply makes all overridden methods match the public
visiblity of the parent.

Differential Revision: https://reviews.llvm.org/D79928