[SLP] Added more missed optimization remarks

Summary:
Added more remarks to SLP pass, in particular "missed" optimization remarks.
Also proposed several tests for new functionality.

Patch by Vladimir Miloserdov!

For reference you may look at: https://reviews.llvm.org/rL302811

Reviewers: anemet, fhahn

Reviewed By: anemet

Subscribers: javed.absar, lattner, petecoup, yakush, llvm-commits

Differential Revision: https://reviews.llvm.org/D38367

llvm-svn: 318307