[mips] Fix a bug for NaCl target - Don't report the error when non-dangerous
load/store is in branch delay slot.

Differential Revision: http://llvm-reviews.chandlerc.com/D4048

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210470 91177308-0d34-0410-b5e6-96231b3b80d8