re PR middle-end/20933 (gcc can no longer bootstrap itself)

	PR tree-optimization/20933
	* tree-ssa-alias.c (compute_flow_insensitive_aliasing): Move
	logic to reject aliases between read-only and writable
	variables ...
	(may_alias_p): ... here.
	(get_tmt_for): Do not associate read-only tags to pointers
	whose pointed-to type is not read-only.
	* tree-ssa.c (verify_ssa): Check that memory stores have at
	least one V_MAY_DEF or V_MUST_DEF.


testsuite/ChangeLog

	PR tree-optimization/20933
	* gcc.dg/tree-ssa/pr20933.c: New test.

From-SVN: r97988