Add specialized handler for checking if identical to string

[TysonAndre]

This has a large performance benefit for the below snippet (0.64s vs 0.94s),
and a small benefit for $alwaysStr == 'notnumeric'/$s = 'other'
(for 1000 runs of test('test'))

• If the literal is non-numeric and the variable is a string,
  == behaves identically to ===, so reuse the opcode.

This is similar to what was suggested in #4900

• Avoiding types with __destruct or notices simplifies the specialized
  implementation and avoids expensive exception handling.
• TMPVARCV doesn't support FREE_OP1() - use TMPVAR|CV instead.

function test(string $s) : int {
    $total = 0;
    for ($i = 0; $i < 100000; $i++) {
        if ($s === "first") {
            $total++;
            $s = null;
        }
    }
    return $total;
}