[RISCV][GISEL] legalize, regbankselect, and instruction-select for G_… (#73061)

…[UN]MERGE_VALUES

When MERGE or UNMERGE s64 on a subtarget that is non-64bit, it must have
the D extension and use FPR in order to be legal.

All other instances of MERGE and UNMERGE that can be made legal should
be narrowed, widend, or replaced by the combiner.