The returned whole and frac must have the same sign as the input parameter.

Tests: dEQP-VK.glsl.builtin.precision.modf.*
Tests: dEQP-VK.glsl.builtin.function.common.modf.*
Bug: b/126873455
Change-Id: I791891fdf46f9285e12d770e2b15b91ebfa3f0b1
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/31590
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>
Presubmit-Ready: Ben Clayton <bclayton@google.com>
Tested-by: Ben Clayton <bclayton@google.com>