gallivm: remove optimization workaround when not having sse 4.1

gallivm: remove optimization workaround when not having sse 4.1

This workaround doesn't list any llvm version, but it was introduced
2010-06-10 (e277d5c1f6b2c5a6d202561e67d2b6821a69ecc4). It is unlikely
this bug is still present in llvm versions we support (3.1+).
There's no specific test listed, but I ran lp_test_arit (which uses
the mentioned functions) on llvm 3.1 and 3.3 with sse41 disabled and
this pass enabled without issues.

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