New pass: PropagateStructGets - #9271

Our main escape analysis pass, Heap2Local, does not handle overlapping
live ranges, things like this:

x = new Foo(42);
if (condition) x = new Foo(1337);
bar(x.value, x.value);

More than one allocation is being read from in those x.value operations,
so Heap2Local sees the uses are not "exclusive", and it gives up.

Handling live ranges, sort of like liveness of locals for register allocation /
local coalescing, is possible, but quite complex. However, the pattern
above can be optimized in another way, which this new pass handles:
when the reference is always read from (using struct.get, which is the
.value operations above), then we can just shift from using the reference
to using data:

x_value = (new Foo(42)).value;
if (condition) x_value = (new Foo(1337)).value;
bar(x_value, x_value);

We store the data in a new local x_value, and that is all we need.

That is, this pass handles a situation that is more restricted than
Heap2Local, in that it doesn't do complex analysis of control flow
and handling of ref.eq and such, but it is more general in that it
can handle overlapping live ranges in a trivial way.

Fixes #8641 , a Dart testcase, and I saw this helps real-world binaries from
Kotlin and Java too - this is a common pattern.