mesa: Replace IS_NEGATIVE(x) with x < 0.0f.

mesa: Replace IS_NEGATIVE(x) with x < 0.0f.

I only made IS_NEGATIVE(x) use signbit in commit 0f3ba405 in an attempt
to fix 54805, but it didn't help. We didn't use signbit on some
platforms and instead defined it to x < 0.0f.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>