SLPVectorizer: Fix invalid iterator errors

Update iterator when the SLP vectorizer changes the instructions in the basic
block by restarting the traversal of the basic block.

Patch by Yi Jiang!

Fixes PR 16899.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188832 91177308-0d34-0410-b5e6-96231b3b80d8