Initialize AsmPrinter::MF in the constructor

MF is normally initialized in AsmPrinter::SetupMachineFunction, but if the file
contains only globals (no functions), then we need this to be initialized
because, when encountering an error, lowerConstant() references it.

This should fix the non-deterministic failures of
test/CodeGen/X86/nonconst-static-iv.ll, etc.