These are identified by misc-include-cleaner.  I've filtered out those
that break builds.  Also, I'm staying away from llvm-config.h,
config.h, and Compiler.h, which likely cause platform- or
compiler-specific build failures.

Add per-property has<Prop>/set<Prop>/reset<Prop> functions to
MachineFunctionProperties.

Contrary to the comment, this particular code is not responsible
for handling any composes that may be required, and unhandled cases
are already rejected later. Lift this restriction to permit composes
and reg_sequence subregisters later.

Subregister defs are illegal in SSA. Surprisingly this enables folding
into subregister insert patterns in one test.

Subregister defs are illegal in SSA

This is a quick fix for EXPENSIVE_CHECKS bot failures. I still think we
could
defer looking for a compatible subregister further up the use-def chain,
and
should be able to check compatibilty with the ultimate found source.

This reverts d246cc618adc52fdbd69d44a2a375c8af97b6106. We now handle
composing subregister extracts through reg_sequence.