gallivm: Prefer backend agnostic intrinsic for rounding.
gallivm: Prefer backend agnostic intrinsic for rounding.

We could unconditionally use these instrinsics, but performance with SSE2
would suck, as LLVM falls back to calling libm.

lp_test_arit.

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

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler