Fix a funny bug I introduced during conversion of ARM constant islands to Mips.

I had to move some code and I moved a declaration forward past it's first use
in the function but by nutty coincidence there was another variable of the same
name and type and  with completely unrelated function that was declared globally
in the class so no compilation error ensued.
It required some unusual conditions for it to even matter. Caused test
case casts.c in test-suite to fail during compilation with a duplicate 
symbol error. I would have noticed it during final code review for this port.

llvm-svn: 195565