AMDGPU: Remove BufferPseudoSourceValue

The use of a PSV for buffer intrinsics is misleading because it may be
misinterpreted as all buffer intrinsics accessing the same address in
memory, which is clearly not true.

Instead, build MachineMemOperands without a pointer value but with an
address space, so that address space-based alias analysis can still
work.

There is a lot of test churn because previously address space 4
(constant address space) was used as an address space for buffer
intrinsics. This doesn't make much sense and seems to have been an
accident -- see the change in
AMDGPUTargetMachine::getAddressSpaceForPseudoSourceKind.

Differential Revision: https://reviews.llvm.org/D138711