Migrate metadata information from scalar to vector instructions during
SLP vectorization. Based on the code in BBVectorizer.

Fixes PR17741.

Patch by Raul Silvera, reviewed by Hal and Nadav. Reformatted by my
driving of clang-format. =]

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@195528 91177308-0d34-0410-b5e6-96231b3b80d8