[LoopVectorize] auto-generate complete checks; NFC

The first test claims to show that the vectorizer will
generate a vector load/loop, but then this file runs
other passes which might scalarize that op. I'm removing 
instcombine from the RUN line here to break that dependency.
Also, I'm generating full checks to make it clear exactly 
what the vectorizer has done.

llvm-svn: 349554