[RISCV][GlobalISel] Legalize bitshift instructions for narrow types

Legalize G_SHL, G_ASHR and G_LSHR for types narrower and upto (and including) XLen: (i7, i8,
i16 and i32) for rv32 and (i8, i15, i16, i32 and i64) for rv64. This requires
adding some rules to handle G_ANYEXT, G_ZEXT and G_SEXT.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D155772