[RISCV][GISEL] Legalize, regbankselect, and instruction-select G_VSCALE (#85967)

G_VSCALE should be lowered using VLENB. If the type is not sXLen it
should be lowered using a G_VSCALE on the narrow type and a G_MUL.
regbank select and instruction select are straightforward so we really
only need to add tests to show it works.