[mips] Expand 'lw/sw' instructions for 32-bit GOT

In case of using 32-bit GOT access to the table requires two instructions
with attached %got_hi and %got_lo relocations. This patch implements
correct expansion of 'lw/sw' instructions in that case.

Differential Revision: https://reviews.llvm.org/D67705

llvm-svn: 372251