[ARM] Sink zext/sext operands for add and sub to enable vsubl generation.

This uses the infrastructure added in rL353152 to sink zext and sexts to
sub/add users, to enable vsubl/vaddl generation when NEON is available.

See https://bugs.llvm.org/show_bug.cgi?id=40025.

Reviewers: SjoerdMeijer, t.p.northover, samparker, efriedma

Reviewed By: samparker

Differential Revision: https://reviews.llvm.org/D58063

llvm-svn: 355460