Treat code following calls to functions returning Never as unreachable.

[mkustermann]

See e.g.

main() {
  foo();
}

@pragma('vm:never-inline')
void foo() {
  int i = 1;
  if (int.parse('1') == 1) {
    i = 2;
    bar(); // <-- Return type `Never`, so it always throws and can never fall through.
  }
  print(i);
}

@pragma('vm:never-inline')
Never bar() {
  throw 'baz';
}

Here the print(i) could be turned into a print(1) but it doesn't seem the VM does that atm:

*** BEGIN CFG
After AllocateRegisters
==== file:///.../foo (RegularFunction)
  0: B0[graph]:0 {
      v0 <- Constant(#null) T{Null?}
      v15 <- UnboxedConstant(#1) [1, 1] int64
      v16 <- UnboxedConstant(#2) [2, 2] int64
}
  2: B1[function entry]:2
  4:     CheckStackOverflow:8(stack=0, loop=0)
  5:     ParallelMove rdi <- C
  6:     v4 <- StaticCall:14( parse<0> v3, result_type = T{int}) [-9223372036854775808, 9223372036854775807] int64
  8:     Branch if EqualityCompare(v4 == v15 T{_Smi}) T{bool} goto (3, 4)
 10: B3[target]:22
 12:     StaticCall:26( bar<0> )
 14:     ParallelMove rax <- C goto:32 B5
 16: B4[target]:28
 18:     ParallelMove rax <- C goto:34 B5
 20: B5[join]:30 pred(B3, B4) {
      v7 <- phi(v16 T{_Smi}, v15 T{_Smi}) alive [1, 2] int64 T{_Smi}
}
 22:     v14 <- BoxInt64(v7) [1, 2] T{_Smi}
 24:     StaticCall:36( print<0> v14 T{_Smi})
 25:     ParallelMove rax <- C
 26:     DartReturn:40(v0)
*** END CFG

[a-siva]

//cc @alexmarkov