[LoopDist] Emit optimization remarks (-Rpass*)

I closely followed the precedents set by the vectorizer:

* With -Rpass-missed, the loop is reported with further details pointing
to -Rpass--analysis.

* -Rpass-analysis reports the details why distribution has failed.

* Regardless of -Rpass*, when distribution fails for a loop where
distribution was forced with the pragma, a warning is produced according
to -Wpass-failed.  In this case the analysis info is also printed even
without -Rpass-analysis.

llvm-svn: 267952