gallivm: Use vector selects on LLVM 3.3+.
gallivm: Use vector selects on LLVM 3.3+.

This is an old patch I had around.

Vector selects seem to work well from LLVM 3.3.  Using them should
improve code quality, as it might make constant propagation pass more
effective.

Tested lp_test_*

Reviewed-by: Roland Scheidegger <7d58aee419d6f9201d75517c885374dff5e7d848@vmware.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler