[GlobalOpt][FIX] Do not embed initializers into AS!=0 globals

Not all address spaces support initializers for globals and we can
therefore not set them without checking if they are allowed. This
patch adds a hook into TTI to check if an AS allows non-undef
initializers. We disable it for all but address space 0 by default,
NVPTX and AMDGPU targets allow all but address space 3.

Reviewed By: tra

Differential Revision: https://reviews.llvm.org/D109337