[SLPVectorizer] Recognise non uniform power of 2 constants

Since D46637 we are better at handling uniform/non-uniform constant Pow2 detection; this patch tweaks the SLP argument handling to support them.

As SLP works with arrays of values I don't think we can easily use the pattern match helpers here.

Differential Revision: https://reviews.llvm.org/D48214

llvm-svn: 335621