[mips] Resolve register classes dynamically using ptr_rc to reduce the number of
load/store instructions defined. Previously, we were defining load/store
instructions for each pointer size (32 and 64-bit), but now we need just one
definition.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188830 91177308-0d34-0410-b5e6-96231b3b80d8