[mips] Remove duplicate tests and add missing prefixes for *-LABEL checks. NFC.

Summary:
The only difference between the removed tests and the pre-existing
ones, is the materialization of the zero constant, which shouldn't
matter for these cases.

Reviewers: dsanders, sdardis

Subscribers: dsanders, sdardis, llvm-commits

Differential Revision: http://reviews.llvm.org/D18693

llvm-svn: 266285