[AMDGPU] Don't combine memory intrs to v3i16

v3i16 and v3f16 currently cannot be legalized and lowered so they should
not be emitted by inst combining.

Moved the check down to still allow extracting 1 or 2 elements via the dmask.

Fixes image intrinsics being combined to return v3x16.

Differential Revision: https://reviews.llvm.org/D84223