[RISCV][GlobalISel] Legalize constants, undefined values, extension...

[RISCV][GlobalISel] Legalize constants, undefined values, extension instructions, and (un)merge instructions for narrow types

Test legalization for (s7, s8, s16, s32, s48, s64, s96) for rv32, (s8, s15, s16, s32, s64, s72, s128, s192) for rv64.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D156383