[SLP] Allow vectorization of the instruction from the same basic blocks only, NFC.

Summary:
After some changes in SLP vectorizer we missed some additional checks to
limit the instructions for vectorization. We should not perform analysis
of the instructions if the parent of instruction is not the same as the
parent of the first instruction in the tree or it was analyzed already.

Subscribers: mzolotukhin

Differential Revision: https://reviews.llvm.org/D34881

llvm-svn: 309425