SLPVectorizer: Bring back the insertelement patch (r205965) with fixes

When can't assume a vectorized tree is rooted in an instruction. The IRBuilder
could have constant folded it. When we rebuild the build_vector (the series of
InsertElement instructions) use the last original InsertElement instruction. The
vectorized tree root is guaranteed to be before it.

Also, we can't assume that the n-th InsertElement inserts the n-th element into
a vector.

This reverts r207746 which reverted the revert of the revert of r205018 or so.

Fixes the test case in PR19621.