Fix hash collision in CSE

[limepoutine]

In cgcs.d, assignment ops clobber CSE of a symbol, but weirdly do not clobber any larger common expressions referencing it. The checks to avoid false matches are a kludge. Instead, this PR now requires all leaf nodes to have already been merged by CSE before eliminating a larger expression. As a result, DMD's CSE is now immune to hash collision (in terms of correctness). This hopefully fixes #20985 (worked for a few hundred builds on my machine yet still unsure).

On the downside, DMD's optimization is slightly weaker again. I think maybe very few people will care.

[limepoutine]

#20985 is much easier to reproduce under low memory like today's Azure pipelines, which very well hints at hash collision as the root cause?

[rainers]

As mentioned in the bug report, the failures are reproducible with cs_comphash() always returning 0, i.e. always causing collisions. Have you tested your change with that?

[limepoutine]

This can pass the whole test suite with hash_t cs_comphash(const elem* e) => 0;. I have no idea how to write a test enforcing that however.

[rainers]

On the downside, DMD's optimization is slightly weaker again. I think maybe very few people will care.

AFAIU only for expressions where the hashes happen to match, which is very unlikely and non-deterministic anyway.

This can pass the whole test suite with hash_t cs_comphash(const elem* e) => 0;.

Good to know.

I have no idea how to write a test enforcing that however.

Probably not worth the trouble. Do we have a test that CSE elimination works at all?

[rainers]

Prepend the commit message with Fix #20985 to auto-close the issue and include it in the changelog.

[limepoutine]

Do we have a test that CSE elimination works at all?

No, I stubbed out all CSE code and tests are still green.

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Testing this PR locally

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dub run digger -- build "master + dmd#22061"