Also fix clamping of the smallest negative value to -1.0.
See https://www.khronos.org/registry/vulkan/specs/1.1/html/vkspec.html#fundamentals-fixedfpconv

Bug: b/167582422
Tests: dEQP-VK.*
Change-Id: Ie77a048c2a3111c19554329e73d81a864f69cf94
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/50948
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>