[RISCV][GlobalISel] Select register banks for GPR ALU instructions

This patch implements the getInstrMapping hook for RISCVRegisterBankInfo and others in order to correctly select the GPR register bank for operands of ALU instructions, and the associated operations introduced by the legalizer.

Co-authored-by: Lewis Revill <lewis.revill@embecosm.com>

Reviewed By: nitinjohnraj

Differential Revision: https://reviews.llvm.org/D76051