[InstCombine] Fold insert sequence if first ins has multiple users.

Summary:
If the first insertelement instruction has multiple users and inserts at
position 0, we can re-use this instruction when folding a chain of
insertelement instructions. As we need to generate the first
insertelement instruction anyways, this should be a strict improvement.

We could get rid of the restriction of inserting at position 0 by
creating a different shufflemask, but it is probably worth to keep the
first insertelement instruction with position 0, as this is easier to do
efficiently than at other positions I think.

Reviewers: grosser, mkuper, fpetrogalli, efriedma

Reviewed By: fpetrogalli

Subscribers: gareevroman, llvm-commits

Differential Revision: https://reviews.llvm.org/D37064

llvm-svn: 312110