Fix justify error for small structures in varargs for MIPS64BE

There was a problem when passing structures as variable arguments.
The structures smaller than 64 bit were not left justified on MIPS64
big endian. This is now fixed by shifting the value to make it left-
justified when appropriate.

This fixes the bug http://llvm.org/bugs/show_bug.cgi?id=21608

Patch by Aleksandar Beserminji.

Differential Revision: http://reviews.llvm.org/D7881

llvm-svn: 230657