Revert "Revert "ARM: Improve pattern for isel mul of vector by scalar.""

This reverts commit r189648.

Fixes for the previously failing clang-side arm_neon_intrinsics test
cases will be checked in separately.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@189841 91177308-0d34-0410-b5e6-96231b3b80d8