[mips] Accept 32-bit offsets for ld/sd/lld commands

This is a follow up to the rL330983. The patch teaches ld, sd, and lld
commands accept 32-bit memory offsets by replacing `mem_simm16` operand
to `mem_simmptr`. In fact, these commands should accept 64-bit offsets,
but so large offsets require another command expanding and will be
supported by a separate patch.

Differential Revision: https://reviews.llvm.org/D46629

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@331997 91177308-0d34-0410-b5e6-96231b3b80d8