Reapply "Mips: Avoid implicit iterator conversions, NFC"

This reverts commit r275562, effectively reapplying r275141.  Doug
Gilmore reported that there was an error when bisecting the Mips
buildbot failure, and that r275141 was not to blame after all.  Here is
the green build:
https://dmz-portal.mips.com/bb/builders/LLVM%20with%20integrated%20assembler%20and%20fPIC%20and%20-O0/builds/803

llvm-svn: 275643