[InstCombine] Fix incorrect Modified status

When removing instructions from unreachable blocks, and only debug info
intrinsics were removed, InstCombine could incorrectly return a false
Modified status.

This is fixed by making removeAllNonTerminatorAndEHPadInstructions()
also return how many debug info intrinsics that were removed, and take
that into account.

This was caught using the check introduced by D80916.

Reviewed By: majnemer

Differential Revision: https://reviews.llvm.org/D85839