Some image ops return three or five dwords. Previously, we modeled that
with a 4 or 8 dword register class. The register allocator could
cleverly spot that some subregs were dead and allocate something else
there, but that caused the de-optimization that waitcnt insertion would
think that the result was used immediately.
This commit allows such an image op to have a result with a three or
five dword result, avoiding the above de-optimization.