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/15608Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>