[MIPS GlobalISel] Silence uninitialized variable warning

The control flow here cannot ever use the uninitialized value, but it's
too hard for the compiler to figure that out. Clang warns:

llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp:2600:28: error: variable 'CarrySum' is used uninitialized whenever 'for' loop exits because its condition is false [-Werror,-Wsometimes-uninitialized]
      for (unsigned i = 2; i < Factors.size(); ++i)
                           ^~~~~~~~~~~~~~~~~~
llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp:2604:26: note: uninitialized use occurs here
    CarrySumPrevDstIdx = CarrySum;
                         ^~~~~~~~
llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp:2600:28: note: remove the condition if it is always true
      for (unsigned i = 2; i < Factors.size(); ++i)
                           ^~~~~~~~~~~~~~~~~~
llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp:2583:22: note: initialize the variable 'CarrySum' to silence this warning
    unsigned CarrySum;
                     ^
                      = 0

llvm-svn: 355818