[AMDGPU] Only set Info.memVT when not later overridden (#92670)

For the amdgcn_*_buffer_load_lds intrinsics this field is later
overriden, so avoid pointlessly calling MVT::getVT in that case.
Importantly, this is also the only case I can find in tree where a
PointerType is passed to MVT::getVT, so this will allow us to forbid
doing so in future, keeping MVT::iPTR as originating solely from
TableGen as was claimed next to its definition in MachineValueType.h
(but lost in the autogeneration conversion).