Preserve full shadow memtransfers

[minansys]

problem

Enzyme was zero-initializing a shadow copy of a struct and then copying only selected active slices from the source shadow. The reverse pass later needed pointer fields from the copied shadow object, but those fields stayed null. The fix preserves a full shadow memtransfer for nonzero memtransfers, so inactive pointer fields remain initialized while existing sliced updates can still run.

Summary

• Preserve the full shadow memtransfer when Enzyme also emits sliced shadow updates, so inactive fields in copied aggregates remain initialized.
• Add a reverse-mode IR regression test covering a copied aggregate with pointer and active scalar fields.

Test plan

• Built LLVMEnzyme-19.so and ClangEnzyme-19.so against the Conan LLVM 19.1.7 package.
• Ran the new standalone IR repro through opt; confirmed the full 32-byte shadow copy is absent without the fix and present with the fix.
• Ran Cromwell TestAdjoint.UnitTestAdjointKW with the minimal fix; it passed.

#2629