[mips] Make checks in CodeGen/mips-varargs.c less fragile

This test was failing on our fork of clang because it was not capturing
[[ARG]] in the N32 case. Therefore it used the value from the last function
which does not always have to be the same. All other cases were already
capturing ARG so this appears to be an oversight.
The test now uses -enable-var-scope to prevent such errors in the future.

Reviewers: sdardis, atanasyan

Patch by: Alexander Richardson

Differential Revision: https://reviews.llvm.org/D32425

llvm-svn: 303619