[mips][atomics] Fix partword atomic binary operation implementation

Currently Mips::emitAtomicBinaryPartword() does not properly respect the
width of pointers. For MIPS64 this causes the memory address that the ll/sc
sequence uses to be truncated. At runtime this causes a segmentation fault.

This can be fixed by applying similar changes as r266204, so that a full 64bit
pointer is loaded.

Reviewers: dsanders

Differential Review: http://reviews.llvm.org/D19651

llvm-svn: 267900