mesa: Drop USE_IEEE define.

mesa: Drop USE_IEEE define.

I think OpenVMS was the only platform that Mesa ran on that used a
non-IEEE representation for floats. We removed OpenVMS support a while
back, and this should alleviate the need to continue updating the
this-platform-uses-IEEE list.

The one bit of this patch that needs review is the IS_INF_OR_NAN,
because I'm not sure if MSVC supports isfinite.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=82268
Reviewed-by: Brian Paul <brianp@vmware.com>