forwprop: Improve the reject case for copy prop [PR107051]

forwprop: Improve the reject case for copy prop [PR107051]

Currently the code rejects:
```
tmp = *a;
*b = tmp;
```
(unless *a == *b). This can be improved such that if a and b are known to
share the same base, then only reject it if they overlap; that is the
difference of the offsets (from the base) is maybe less than the size.

This fixes the testcase in comment #0 of PR 107051.

Changes since v1:
* v2: Use ranges_maybe_overlap_p instead of manually checking the overlap.
      Allow for the case where the alignment is known to be greater than
      the size.

	PR tree-optimization/107051

gcc/ChangeLog:

	* tree-ssa-forwprop.cc (optimize_agr_copyprop_1): Allow for
	memory sharing the same base if they known not to overlap over
	the size.

gcc/testsuite/ChangeLog:

	* gcc.dg/tree-ssa/copy-prop-aggregate-union-1.c: New test.

Signed-off-by: Andrew Pinski <andrew.pinski@oss.qualcomm.com>