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

[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@372824 91177308-0d34-0410-b5e6-96231b3b80d8