[GlobalOpt] Fix memset handling in global ctor evaluation (PR55859)

The global ctor evaluator currently handles by checking whether the
memset memory is already zero, and skips it in that case. However,
it only actually checks the first byte of the memory being set.

This patch extends the code to check all bytes being set. This is
done byte-by-byte to avoid converting undef values to zeros in
larger reads. However, the handling is still not completely correct,
because there might still be padding bytes (though probably this
doesn't matter much in practice, as I'd expect global variable
padding to be zero-initialized in practice).

Mostly fixes https://github.com/llvm/llvm-project/issues/55859.

Differential Revision: https://reviews.llvm.org/D128532