More conservatively report status from LoopIdiomRecognize

Being "precise" here is getting us into trouble with one of the
EXPENSIVE_CHECKS buildbots, see [1].  Rather than reporting IR additions that
later get rolled back as "no change", instead we now conservatively report that
there was.

1: http://lists.llvm.org/pipermail/llvm-dev/2020-July/143509.html

Differential Revision: https://reviews.llvm.org/D84071