gallivm: (trivial) simplify lp_build_cos/lp_build_sin a tiny bit

gallivm: (trivial) simplify lp_build_cos/lp_build_sin a tiny bit

Use "or" instead of "add" (this is a classic select sequence, which at
least newer llvm versions can actually recognize (3.2+?), and the "add"
might prevent that - and we really don't want an add instead of an or with
avx if it isn't recognized (even without avx logic ops might be cheaper)).

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>