Fix list assignment with undef placeholders

[fglock]

Fixes list assignment semantics when the LHS contains placeholders.

This restores behavior needed by perl5_t/t/op/aassign.t:

• "undef as lvalue" (test 176)
• GH #17816 (test 188)
• GH #16685 (test 189)

Approach:

• Preserve RHS aliases before consuming the RHS list.
• For placeholders on the LHS, return an RHS alias when safe, otherwise return the pre-assignment RHS value.
• Ensure placeholders still contribute to the returned list length.