[Nios2] Arithmetic instructions for R1 and R2 ISA.

Summary:
This commit enables some of the arithmetic instructions for Nios2 ISA (for both
R1 and R2 revisions), implements facilities required to emit those instructions
and provides LIT tests for added instructions.

Reviewed By: hfinkel

Differential Revision: https://reviews.llvm.org/D41236

Author: belickim <mateusz.belicki@intel.com>
llvm-svn: 322069