refactor SSA rewriting timevars

refactor SSA rewriting timevars

This avoids too deep stacks of timevars during incremental
SSA rewrite and basically use TV_TREE_INTO_SSA for all into-SSA
rewrite work and TV_TREE_SSA_INCREMENTAL for update_ssa.

2021-05-06  Richard Biener  <rguenther@suse.de>

	* timevar.def (TV_TREE_INSERT_PHI_NODES): Remove.
	(TV_TREE_SSA_REWRITE_BLOCKS): Likewise.
	(TV_TREE_INTO_SSA): New.
	* tree-into-ssa.c (insert_phi_nodes): Do not account separately.
	(rewrite_blocks): Likewise.
	(pass_data_build_ssa): Account to TV_TREE_INTO_SSA.