Now `lw/sw $reg, sym+offset` pseudo instructions for global symbol `sym`
are lowering into the following three instructions.
```
lw $reg, %got(symbol)($gp)
addiu $reg, $reg, offset
lw/sw $reg, 0($reg)
```
It's possible to reduce the number of instructions by taking the offset
in account in the final `lw/sw` command. This patch implements that
optimization.
```
lw $reg, %got(symbol)($gp)
lw/sw $reg, offset($reg)
```
Differential Revision: https://reviews.llvm.org/D66553
llvm-svn: 369756