Remove the unit test from r321783.
This test fails when run on the sanitizer bot, and I do not see a good
way to fix it. The existing bogus target in MachineInstrTest.cpp is only
good enough to create instructions but not sufficient to insert them into
basic blocks. The addNodeToList ilist callback dereferences the pointer
to the MachineRegisterInfo. Adding MachineRegisterInfo would also require
TargetRegisterInfo, even a minimal implementation of that would be quite
complicated. I would be glad to add this back if someone can suggest a
better way to do it.