Scalarize select vector arguments when extracted.

When the elements are extracted from a select on vectors
or a vector select, do the select on the extracted scalars
from the input if there is only one use.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194013 91177308-0d34-0410-b5e6-96231b3b80d8