[NewPM] fixing asserts on deleted loop in -print-after-all

[NewPM] fixing asserts on deleted loop in -print-after-all

IR-printing AfterPass instrumentation might be called on a loop
that has just been invalidated. We should skip printing it to
avoid spurious asserts.

Reviewed By: chandlerc, philip.pfaffe
Differential Revision: https://reviews.llvm.org/D54740

llvm-svn: 348887