AllocationSinking issues with records and closures

[mraleph]

class A {
  int? f1;
  double? f2;

  @pragma('vm:never-inline')
  String foo() {
    return switch ((this.f1, this.f2)) {
      (final f1, _) => '$f1',
      (null, final f2) => '$f2',
      (null, null) => '?',
    };
  }
}

void main() {
  print(A().foo());
}

results in

*** BEGIN CFG
After AllocateRegisters
==== file:///tmp/test.dart_A_foo (RegularFunction)
  0: B0[graph]:0 {
      v0 <- Constant(#null) T{Null?}
      v5 <- Constant(#Function '##0#1#initializer':.) T{Function}
      v7 <- Constant(#sentinel) T{Sentinel~}
}
  2: B9[function entry]:2 {
      v2 <- Parameter(0) T{A}
}
  4:     CheckStackOverflow:8(stack=0, loop=0)
  6:     v3 <- AllocateContext:12(num_variables=3, <not-aliased>) T{Context}
  7:     ParallelMove rbx <- C, rdx <- C, rax <- rax
  8:     ParallelMove S-1 <- rax
  8:     v4 <- AllocateSmallRecord:18(v0, v0) T{_Record}
  9:     ParallelMove rcx <- rax, rax <- S-1
 10:     ParallelMove S-2 <- rcx
 10:     StoreField(v3 . #0#0 = v4, NoStoreBarrier)
 11:     ParallelMove rbx <- C, rdx <- rax
 12:     v6 <- AllocateClosure:24(v5, v3) T{_Closure}
 13:     ParallelMove rcx <- rax, rax <- S-1
 14:     StoreField(v3 . ##0#1#initializer = v6, NoStoreBarrier)
 15:     ParallelMove rcx <- C
 16:     StoreField(v3 . #0#1 = v7, NoStoreBarrier)
 17:     ParallelMove rcx <- S-2
 18:     v114 <- LoadField(v4 T{_Record} . :record_field) T{int??}
 20:     StoreField(v3 . #0#1 = v114, NoStoreBarrier)
 22:     MoveArgument(v114 T{int??}, SP+0)
 24:     v34 <- StaticCall:74( _interpolateSingle@0150898<0> v114 T{int??}) T{String}
 26:     Return:276(v34 T{String})
*** END CFG

Two problems here:

• Initial value (null) of the record field is not forwarded. Probably due to limitations around forwarding of initial null from final fields - these limitations should not apply to records. And strictly speaking they could be lifted from final fields as well if we are a bit smarter with expressing side-effects: only calls to constructors can initialize final fields of normal objects.
• Closure v6 is not sunk probably because it's context is referencing itself. We could be smarter around that. This code should have 0 allocations.

Note: issues with using late variables for patterns lowering are tracked in a separate issue #52805

/cc @alexmarkov can you take a look?

[alexmarkov]

Flow graph after patterns lowering stopped using late variables in 85cb56d

*** BEGIN CFG
After AllocateRegisters
==== file:///.../foo.dart_A_foo (RegularFunction)
  0: B0[graph]:0 {
      v52 <- Constant(#null) T{_OneByteString}
}
  2: B7[function entry]:2 {
      v2 <- Parameter(0) T{A}
} 
  3:     ParallelMove rax <- C
  4:     Return:252(v52)
*** END CFG