[PATCH] have -Warray-bounds treat [max, min] ranges same as anti-ranges (PR 89720)

[Martin Sebor]

I the -Warray-bounds enhancement committed at the beginning
of the GCC 9 window I tried to correctly handle offsets in
MEM_REFs in the [max, min] kind of a range after converting
from sizetype to ptrdiff_type, but I did it wrong.  The bug
results in false positives in some unusual use cases that
I didn't consider at the time.

The attached patch removes this incorrect handling and uses
the conservative anti-range handling for these cases instead.

Martin

PS Is there some technical reason why pointer offsets are
represented as the unsigned sizetype when they can be signed?
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