Preserve escaping uses before promoting other allocas

When the destination alloca is visited before the source alloca, this per-alloca position check is order-dependent: for %b = alloca 32; %a = alloca 32; mstore %b, %a, %b passes this check as the address operand and line 86 rewrites the mstore away before %a is examined. %a then no longer has its value-operand escape in the DFG and can still be promoted/deleted, so the issue remains for allocas declared after their destination slot. The escape set needs to be determined from the original uses before mutating stores, or stores of any alloca pointer should block promotion that would remove them.

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Verified empirically — the order-swapped shape is handled correctly. With %b visited first:

%b = alloca 32
%a = alloca 32
mstore %b, %a
%x = mload %a
%y = mload %b
sink %x, %y

promoting %b rewrites the mstore to %alloca_b_0 = %a via InstUpdater, which maintains the DFG — so when %a is examined, its uses are [assign, mload], and the assign opcode fails _is_pointer_use, blocking promotion. Output after MakeSSA+Mem2Var:

%b = alloca 32
%a = alloca 32
%alloca_b_0 = %a
%x = mload %a
%y = %alloca_b_0
sink %x, %y

%a survives as a real allocation with a real load, and the pointer value flows into %b's replacement variable — which is exactly the promotion semantics for %b (its slot held the pointer value). Note the escape check exists to prevent the pointer bytes from being silently dropped, not to prevent the destination slot from being promoted; once the destination is a stack variable, the stored pointer value is preserved in SSA.

Added test_mem2var_stored_pointer_dest_promoted_first (722a274) pinning this shape.