[mips] Accept 32-bit offsets for lh and lhu commands

This is a follow up to the rL330983. The patch teaches lh and lhu
commands accepts 32-bit memory offsets by replacing `mem_simm16` operand
to `mem_simmptr`.

Differential Revision: https://reviews.llvm.org/D46513

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@331996 91177308-0d34-0410-b5e6-96231b3b80d8