MipsISelLowering.cpp: Fill unreachable paths to fix warnings. [-Wsometimes-uninitialized]

FIXME: Could they, unreachable(s), be removed?
FIXME: I could prefer the coding standards...

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173325 91177308-0d34-0410-b5e6-96231b3b80d8