[RISCV] Add support for all RV32I instructions

This patch supports all RV32I instructions as described in the RISC-V manual.
A future patch will add support for pseudoinstructions and other instruction
expansions (e.g. 0-arg fence -> fence iorw, iorw).

Differential Revision: https://reviews.llvm.org/D23566

llvm-svn: 313485