Fix the Windows build in GlobalISel

Clang-cl was complaining about some sort of constexpr narrowing bug:

C:\src\llvm-project\llvm\lib\CodeGen\GlobalISel\CombinerHelper.cpp(136,31):  error: non-constant-expression cannot be narrowed from type 'llvm::TargetOpcode::(anonymous enum at C:\src\llvm-project\llvm\include\llvm/CodeGen/TargetOpcodes.h:22:1)' to 'unsigned int' in initializer list [-Wc++11-narrowing]
                              unsigned(MI.getOpcode()) == unsigned(TargetOpcode::G_LOAD)
                              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
C:\src\llvm-project\llvm\lib\CodeGen\GlobalISel\CombinerHelper.cpp(136,31):  note: insert an explicit cast to silence this issue
                              unsigned(MI.getOpcode()) == unsigned(TargetOpcode::G_LOAD)
                              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                              static_cast<unsigned int>(

llvm-svn: 343541