i965: Move IF stack handling into the EU abstraction layer/brw_compile.

i965: Move IF stack handling into the EU abstraction layer/brw_compile.

This hides the IF stack and back-patching of IF/ELSE instructions from
each of the code generators, greatly simplifying the interface.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>