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Restore callee-saved registers via rename to preserve zone edges

When a local call returns, the old restore logic used individual
assign+havoc operations to move saved register values back. Each
assign could lose zone relationships (via the singleton shortcut in
ZoneDomain::assign), destroying edges between loop counters and
callee-saved registers.

Replace the restore with a bulk rename: relabel the frame variables
back to register names in both ZoneDomain (vert_map/rev_map swap)
and TypeDomain (VarIdMap swap). The SplitDBM graph is untouched —
all zone edges survive, including counter relationships.

The save remains a copy (assign), not a rename, so that the callee
can still read the caller's r6-r9 per the BPF calling convention.
Also remove the is_top() skip in save — a variable can have useful
zone edges (relational constraints) even when its interval is top.

Fixes #1119, related to #670.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
Signed-off-by: Elazar Gershuni <elazarg@gmail.com>

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