[RISCV][GISel] Add instruction selection for G_SEXT, G_ZEXT, and G_SEXT_INREG (#67359)

G_SEXT and G_ZEXT are supported via patterns imported from SDISel;
G_SEXT_INREG is selected using hand-written code as there is no
(functional) rule at this moment to import G_SEXT_INREG from
ISD::SEXT_INREG.

Credit helps from @topperc on G_SEXT and G_ZEXT.