Creating variables makes statement get removed even with `self`

[evantypanski]

The following HILTI program has different behavior with optimizations and without:

module Foo {

import hilti;

type Test = struct() {
    method int<64> test();
    int<64> x;
};

method int<64> Test::test() {
    self.x = 5;
    return 1;
}

function void x() {
    local Test dat = [$x = 1];
    hilti::print(dat);
    local y = dat.test();
    hilti::print(dat);
}

x();
}

etyp:/Users/etyp/src/spicy jj:(r:zzw) $ hiltic -j test.hlt -g
[$x=1]
[$x=5]
etyp:/Users/etyp/src/spicy jj:(r:vtw) $ hiltic -j test.hlt
[$x=1]
[$x=1]

note that this is just because y is never marked as used so it gets rid of the statement. If you just do dat.test(); then it doesn't get removed.

I'm hitting this when finishing #2228, a test is failing because a method gets removed, and it gets removed despite using self (as a method call).

[bbannier]

This broke with #2161 which would be new in spicy-1.15. Since this breaks valid code I'd consider this a blocker.