[AArch64] Convert neon_ushl and neon_sshl with positive constants to VSHL.

I think we should be able to use shl instead of sshl and ushl for
positive constant shift values, unless I am missing something.

We already have the machinery in place to ensure we only replace
nodes, if the shift value is positive and <= the element width.

This is a generalization of an earlier patch rL372565.

Reviewers: t.p.northover, samparker, dmgreen, anemet

Reviewed By: anemet

Differential Revision: https://reviews.llvm.org/D67955

llvm-svn: 372824