llvm-reduce: Support multiple MachineFunctions

The current testcase I'm trying to reduce only reproduces with IPRA
enabled and requires handling multiple functions.

The only real difference vs. the IR is the extra indirect to look for
the underlying MachineFunction, so treat the ReduceWorkItem as the
module instead of the function.

The ugliest piece of this is really the ugliness of
MachineModuleInfo. It not only tracks actual module state, but has a
number of transient fields used for isel and/or the asm printer. These
shouldn't do any harm for the use here, though they should be
separated out.