Don't perform very early depth test when overridden by shader.

Don't perform very early depth test when overridden by shader.

When the pixel shader writes the depth value, we can't use the
interpolated implicit depth to perform a depth buffer test before the
pixel shader executes.

Change-Id: Icf80de50f4aa79f73b43e341c18478e7fd928b8d
Reviewed-on: https://swiftshader-review.googlesource.com/15608
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>