[AMDGPU] Fix for issue in alloca to vector promotion pass

Summary:
Alloca promotion pass not dealing with non-canonical input

Added some additional checks so the pass simply backs-off forms it can't deal with (non-canonical)

Also added some test cases in non-canonical form to check that it no longer crashes

Subscribers: arsenm, kzhuravl, wdng, nhaehnle, yaxunl, tpr, t-tye

Differential Revision: https://reviews.llvm.org/D31710

llvm-svn: 305079