[InstCombine] Allow values with multiple users in SimplifyDemandedVectorElts

Summary:
Allow for ignoring the check for a single use in SimplifyDemandedVectorElts
to be able to simplify operands if DemandedElts is known to contain
the union of elements used by all users.
It is a responsibility of a caller of SimplifyDemandedVectorElts to
supply correct DemandedElts.

Simplify a series of extractelement instructions if only a subset of
elements is used.

Reviewers: reames, arsenm, majnemer, nhaehnle

Reviewed By: nhaehnle

Subscribers: wdng, jvesely, nhaehnle, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D67345

llvm-svn: 375395