[MIPS] Fix justify error for small structures

Fix justify error for small structures bigger than 32 bits in fixed
arguments for MIPS64 big endian. There was a problem when small structures
are passed as fixed arguments. The structures that are bigger than 32 bits
but smaller than 64 bits were not left justified properly on MIPS64 big
endian. This is fixed by shifting the value to make it left justified when
appropriate.

Patch by Aleksandar Beserminji.

Differential Revision: http://reviews.llvm.org/D8174

llvm-svn: 232382