Fix string coercion for $a .= $a (#11296)

free_op2_string may be set to false when the operands are not strings, and
`result == op1 == op2`, by re-using the same string for both operands

Fix string coercion for $a .= $a (#11296)

free_op2_string may be set to false when the operands are not strings, and
`result == op1 == op2`, by re-using the same string for both operands. In that
case, the string should still be copied to result because result is not actually
a string. Also change the op1 branch to stay consistent.

Introduced by GH-10049

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