llvmpipe: use sse2 conv code for altivec

llvmpipe: use sse2 conv code for altivec

In lp_build_conv() and lp_build_conv_auto(), there is a special case of
conversion when sse2 is present. That code path is suitable without any
changes to altivec, because all the functions that are called in that
code path already support altivec.

This patch increase the FPS in POWER arch across the board
between 10%-25%

I checked ipers, glxgears, glxspheres64, openarena, xonotic and glmark2.

Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>