mesa: Don't flatten IF statements by default.

mesa: Don't flatten IF statements by default.

MaxIfDepth of 0 means "flatten all the time", not "never flatten".
This is only desirable on hardware that can't support control flow;
software rasterization and most hardware drivers want this.

This alters behavior for swrast as well as i915.  Tested on i915.

NOTE: This is a candidate for stable release branches.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>