X86: stifle GCC warning

lib/Target/X86/X86TargetTransformInfo.cpp: In member function ‘virtual unsigned int {anonymous}::X86TTI::getIntImmCost(unsigned int, unsigned int, const llvm::APInt&, llvm::Type*) const’:
lib/Target/X86/X86TargetTransformInfo.cpp:920:60: warning: enumeral and non-enumeral type in conditional expression [enabled by default]

This seems like an unhelpful warning, but there doesnt seem to be a controlling
flag, so add an explicit cast to silence the warning.