Z was only being interpolated when the Position builtin is used (which
isn't valid for fragment shaders) and/or when there's a depth buffer
present. Instead interpolate it when FragCoord is being used.

Bug: b/133325267
Tests: dEQP-VK.draw.inverted_depth_ranges.*
Change-Id: I0567084b6f7bb054bd864545fa34acc52c8ac3d6
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/31848
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>