[LV] Inform about exactly reason of loop illegality

Currently, only the following information is provided by LoopVectorizer
in the case when the CF of the loop is not legal for vectorization:

 LV: Can't vectorize the instructions or CFG
    LV: Not vectorizing: Cannot prove legality.

But this information is not enough for the root cause analysis; what is
exactly wrong with the loop should also be printed:

 LV: Not vectorizing: The exiting block is not the loop latch.

Patch by Pavel Samolysov.

Reviewers: mkuper, hsaito, rengolin, fhahn

Reviewed By: fhahn

Differential Revision: https://reviews.llvm.org/D62311

llvm-svn: 362056