Unify the logic in SelectAtomicLoadAdd and SelectAtomicLoadArith

- Merge the processing of LOAD_ADD with other atomic load-arith
  operations
- Separate the logic getting target constant for atomic-load-op and add
  an optimization for atomic-load-add on i16 with negative value
- Optimize a minor case for atomic-fetch-add i16 with negative operand. Test
  case is revised.

llvm-svn: 164243