[PATCH] avoid assuming known string length is constant (PR 83896)

[Martin Sebor]

Recent improvements to the strlen pass introduced the assumption
that when the length of a string has been recorded by the pass
the length is necessarily constant.  Bug 83896 shows that this
assumption is not always true, and that GCC fails with an ICE
when it doesn't hold.  To avoid the ICE the attached patch
removes the assumption.

x86_64-linux bootstrap successful, regression test in progress.

Martin
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