AMDGPU: Remove ImagePSV and move images to addrspace 7

Following up on the removal of BufferPSV in commit 43b86bf9 ("AMDGPU:
Remove BufferPseudoSourceValue")

It is unclear what exactly the right address space for images should be.
They seem morally closest to buffers, so that's what I went with. In
practical terms, address space 7 is better than address space 0 because
it can't alias with LDS.

Differential Revision: https://reviews.llvm.org/D138949