Previously if sample count were 1, we would skip the setup for the
centroid 1/w, and then blindly use it, leading to NaN in all interpolants marked `centroid`.

Bug: b/118386749
Test: dEQP-VK.glsl.linkage.varying.*
Change-Id: I4f5d0b052f2defaad86706b85790ef14cd42f53f
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/29650Tested-by: Chris Forbes <chrisforbes@google.com>
Presubmit-Ready: Chris Forbes <chrisforbes@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>