gallivm: handle nan's in min/max

gallivm: handle nan's in min/max

Both D3D10 and OpenCL say that if one the inputs is nan then
the other should be returned. To preserve that behavior
the patch fixes both the sse and the non-sse paths in both
functions and adds helper code for handling nans.

Signed-off-by: Zack Rusin <zackr@vmware.com>
Reviewed-by: Jose Fonseca <jfonseca@vmware.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>