(Trivial) TargetPassConfig: assert when TargetMachine has no MCAsmInfo
This is a pretty trivial, but I thought it was worth just checking that nobody feels it's completely the wrong thing to be doing.
The motivation is that when starting a new backend, you often start with a minimal stub, pretty much just FooTargetMachine and FooTargetInfo. Once that's built, you might naturally try llc -march=foo myinput.ll and it seems more developer-friendly if this ends up asserting due to the lack of MCAsmInfo with an informative message rather than just segfaulting.
Reviewers: MatzeB, chandlerc
Subscribers: bogner, llvm-commits
Differential Revision: https://reviews.llvm.org/D23443