rtl-optimization/98144 - tame REE memory usage

rtl-optimization/98144 - tame REE memory usage

This changes the REE dataflow to change the explicit all-ones
starting solution to be implicit via a visited flag, removing
the need to initially start with fully populated bitmaps for
all basic-blocks.  That reduces peak memory use when compiling
the RTL checking enabled insn-extract.c testcase from PR98144
from 6GB to less than 2GB.

2021-01-29  Richard Biener  <rguenther@suse.de>

	PR rtl-optimization/98144
	* df.h (df_mir_bb_info): Add con_visited member.
	* df-problems.c (df_mir_alloc): Initialize con_visited,
	do not fully populate IN and OUT.
	(df_mir_reset): Likewise.
	(df_mir_confluence_0): Set con_visited.
	(df_mir_confluence_n): Properly handle implicitely
	fully populated IN and OUT as designated by con_visited
	and update con_visited accordingly.