fixes #18238; Nested object construction can zero same memory multiple times for --mm:refc

[ringabout]

fixes #18238

This pull request makes a targeted change to the object construction logic in the genObjConstr procedure. The main update refines the conditions under which memory zeroing is required during object construction, making the behavior more accurate for different garbage collection and destructor options.

Key logic update:

• Improved the needsZeroMem condition in genObjConstr to check for the presence of garbage-collected references and the optSeqDestructors option, instead of relying solely on the selected garbage collector and field flags. This ensures memory is zeroed only when necessary, potentially improving performance and correctness.

T1_ = NIM_NIL;
T1_ = ((tyObject_E__uEKympBdEK4SY9anUbpNaLQ*) newObj((&NTIrefe__bJ9cSuxv8xHYxmdolQqFkUw_), sizeof(tyObject_E__uEKympBdEK4SY9anUbpNaLQ)));
nimZeroMem(((void*) ((&(*T1_).z.z.z.z))), sizeof(tyObject_A__G2lWlL9cFqoiWWwZmWqfJ9bA));
(*T1_).z.z.z.z.y = ((NI) 5);
asgnRef(((void**) ((&z1__test8_u12))), T1_);
asgnRef(((void**) ((&z2__test8_u55))), new__test8_u13());
(*z2__test8_u55).z.z.z.z.y = ((NI) 5);
T2_ = NIM_NIL;
T2_ = ((tyObject_E__uEKympBdEK4SY9anUbpNaLQ*) newObj((&NTIrefe__bJ9cSuxv8xHYxmdolQqFkUw_), sizeof(tyObject_E__uEKympBdEK4SY9anUbpNaLQ)));
asgnRef(((void**) ((&z3__test8_u56))), T2_);
(*z3__test8_u56).z.z.z.z.y = ((NI) 5);

The original test case has already been fixed for ORC, now extends it to refc: if a constructor is fully initialized, it does not need a zero-fill step

[Copilot]

Pull request overview

This PR adjusts the C backend’s object-construction codegen (genObjConstr) to reduce redundant zero-initialization during nested object construction, targeting the --mm:refc case described in issue #18238.

Changes:

• Refines the needsZeroMem condition in compiler/ccgexprs.nim to avoid unconditional zeroing based solely on the selected GC, and instead incorporate containsGarbageCollectedRef(t) plus the optSeqDestructors setting.

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