AMDGPU/SI: Extend promoting alloca to vector to arrays of up to 16 elements

Summary:
  This patch extends the promotion of alloca to vector to the arrays of up to 16 elements. Also we introduce
an option, -disable-promote-alloca-to-vector, to switch promotion to vector off, if needed.

Reviewers:
  arsenm

Differential Revision:
  https://reviews.llvm.org/D33559

llvm-svn: 325372