tree-optimization/99824 - avoid excessive integer type precision in VN

tree-optimization/99824 - avoid excessive integer type precision in VN

VN sometimes builds new integer types to handle accesss where precision
of the access type does not match the access size.  The way
ao_ref_init_from_vn_reference is computing the access size ignores
the access type in case the ref operands have an outermost
COMPONENT_REF which, in case it is an array for example, can be
way larger than the access size.  This can cause us to try
building an integer type with precision larger than WIDE_INT_MAX_PRECISION
eventually leading to memory corruption.

The following adjusts ao_ref_init_from_vn_reference to only lower
access sizes via the outermost COMPONENT_REF but otherwise honor
the access size as specified by the access type.

It also places an assert in integer type building that we remain
in the limits of WIDE_INT_MAX_PRECISION.  I chose the shared code
where we set TYPE_MIN/MAX_VALUE because that will immediately
cross the wide_ints capacity otherwise.

2021-03-30  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/99824
	* stor-layout.c (set_min_and_max_values_for_integral_type):
	Assert the precision is within the bounds of
	WIDE_INT_MAX_PRECISION.
	* tree-ssa-sccvn.c (ao_ref_init_from_vn_reference): Use
	the outermost component ref only to lower the access size
	and initialize that from the access type.

	* gcc.dg/torture/pr99824.c: New testcase.