[committed] avoid -Wrestrict on structs copies with unknown offset and constant size (PR 86196)

[Martin Sebor]

A fix for pr85753 I committed back in may introduced a subtle
logic error that cause false positives for copies involving
unknown offsets into non-array objects.  The attached patch
corrects this mistake.  I have committed it as obvious in
r266967 after verifying it on x86_64-linux.  The committed patch
is attached.

Martin
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