[WGSL] Overload resolution shouldn't eagerly promote variables

[tadeuzagallo]

5186c24

[WGSL] Overload resolution shouldn't eagerly promote variables
https://bugs.webkit.org/show_bug.cgi?id=255771
rdar://108361197

Reviewed by Mike Wyrzykowski.

During overload resolution, when trying to assign a type to a variable we check
whether it satisifies the variable's constraints. In order to satisfy a constraint
we might need to promote the type being assigned to the variable. The example
from the tests is when the variable requires a concrete type but we are assigning
an abstract type to it. Originally, we would promote the type as soon as we assigned
it to the variable, but that is not always correct. The promotion algorithm will
pick the cheapest conversion, in this case it would promote an AbstractInt to an
i32, which is a valid conversion. However, later we observe that the same variable
is also being unified with an u32, which was also a valid conversion for the original
type (AbstractInt), but since we already promoted the variable to i32, we can no
longer unify the variable with u32, since we can't unify i32 and u32. The solution
is faily straightforward though: we still reject types that don't satisfy the constraints,
but hold off on promoting the type until we materialize the variable after determining
that a given overload is a viable candidate. This guarantees that there will be a
suitable promotion that will satisfy the constraints, but we delay choosing which
type we'll promote to until we have looked at all arguments.

* Source/WebGPU/WGSL/Overload.cpp:
(WGSL::OverloadResolver::materialize const):
(WGSL::OverloadResolver::assign):
* Source/WebGPU/WGSL/tests/valid/overload.wgsl:

Canonical link: https://commits.webkit.org/263355@main

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[djg]

[webkit-early-warning-system]

EWS run on current version of this PR (hash 37d466f)

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	✅ 🛠 ios-sim	✅ 🛠 mac-AS-debug	🧪 wpe-wk2
✅ 🧪 webkitperl	✅ 🧪 ios-wk2	✅ 🧪 api-mac	✅ 🛠 gtk
	✅ 🧪 ios-wk2-wpt	✅ 🧪 mac-wk1	🧪 gtk-wk2
	✅ 🧪 api-ios	✅ 🧪 mac-wk2	✅ 🧪 api-gtk
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[webkit-commit-queue]

Committed 263355@main (5186c24): https://commits.webkit.org/263355@main

Reviewed commits have been landed. Closing PR #13017 and removing active labels.