Investigate why += is compiled so inefficiently in compiled mode

[perlun]

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While working on finalizing #415, I saw something interesting. The following Perlang program (https://github.com/perlang-org/perlang/blob/52f7d7d303eba83fa3fe3bdb5e9725aa940dee0f/docs/examples/quickstart/pi.per):

// pi.per
//
// The number of digits we should calculate
var digits = 10000;

var i = 1;
var x = 3 * (10 ** (digits + 20));
var pi = x;

while (x > 0) {
    x = x * i / ((i + 1) * 4);
    pi += (x / (i + 2));
    i += 2;
}

print(pi / (10 ** 20));

...gets compiled into this:

// Automatically generated code by Perlang 0.4.0-dev.20 at 2024-02-24T20:15:53.3599921Z
// Do not modify. Changes to this file might be overwritten the next time the Perlang compiler is executed.

#include <math.h> // fmod()
#include <stdint.h>

#include "bigint.hpp" // BigInt
#include "stdlib.hpp"

//
// Method definitions
//
int main();

//
// Method declarations
//
int main() {
    int32_t digits = 10000;
    int32_t i = 1;
    BigInt x = (BigInt)3 * (perlang::BigInt_pow(10, (digits + 20)));
    BigInt pi = x;
    while (x > 0)     {
        x = x * (BigInt)i / ((i + 1) * 4);
        pi = pi += (x / (i + 2));
        i = i += 2;
    }
;
    perlang::print((pi / (perlang::BigInt_pow(10, 20))));
}

Note the pi = pi += (x / (i + 2)). Why on earth is this? It should be enough with just pi += (x / (i + 2)), I believe.