Changes

[XRay][test] Fix xray-empty-firstmbb.mir and delete incorrect
xray-empty-function.mir
xray-empty-firstmbb.mir does not test the intended code path. Change 
xray-instruction-threshold to 0 to exercise the code path.
Delete xray-empty-function.mir . Empty MachineFunction does not work. 
Various passes (e.g. MachineDominatorTree) assume the presence of an 
entry block.

[NFC] Make the type of X86AlignBranchBoundary compatible
Change the type of X86AlignBranchBoundary from cl::opt<uint64_t> to 
cl::opt<unsigned> since the template class cl::opt is only instantiated
with type unsigned, int, std::string, char and bool.