[AArch64] support neon_sshl and neon_ushl in performIntrinsicCombine.

Try to generate ushll/sshll for aarch64_neon_ushl/aarch64_neon_sshl,
if their first operand is extended and the second operand is a constant

Also adds a few tests marked with FIXME, where we can further increase
codegen.

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

Reviewed By: anemet

Differential Revision: https://reviews.llvm.org/D62308

llvm-svn: 372565