Closes#2535
See the referenced issue for longer discussion - here's the synopsis.
Consider this program
```
app "test" provides [ nums ] to "./platform"
alpha = { a: 1, b: 2 }
nums : List U8
nums =
[
alpha.a,
alpha.b,
]
```
Here's its IR:
```
procedure : `#UserApp.alpha` {I64, U8}
procedure = `#UserApp.alpha` ():
let `#UserApp.5` : Builtin(Int(I64)) = 1i64;
let `#UserApp.6` : Builtin(Int(U8)) = 2i64;
let `#UserApp.4` : Struct([Builtin(Int(I64)), Builtin(Int(U8))]) = Struct {`#UserApp.5`, `#UserApp.6`};
ret `#UserApp.4`;
procedure : `#UserApp.nums` List U8
procedure = `#UserApp.nums` ():
let `#UserApp.7` : Struct([Builtin(Int(I64)), Builtin(Int(U8))]) = CallByName `#UserApp.alpha`;
let `#UserApp.1` : Builtin(Int(U8)) = StructAtIndex 1 `#UserApp.7`;
let `#UserApp.3` : Struct([Builtin(Int(I64)), Builtin(Int(U8))]) = CallByName `#UserApp.alpha`;
let `#UserApp.2` : Builtin(Int(U8)) = StructAtIndex 1 `#UserApp.3`;
let `#UserApp.0` : Builtin(List(Builtin(Int(U8)))) = Array [`#UserApp.1`, `#UserApp.2`];
ret `#UserApp.0`;
```
What's happening is that we need to specialize `alpha` twice - once for the
type of a narrowed to a U8, another time for the type of b narrowed to a U8.
We do the specialization for alpha.b first - record fields are sorted by
layout, so we generate a record of type {i64, u8}. But then we go to
specialize alpha.a, but this has the same layout - {i64, u8} - so we reuse
the existing one! So (at least for records), we need to include record field
order associated with the sorted layout fields, so that we don't reuse
monomorphizations like this incorrectly!
While building the main function for tests, we may need to generate
additional blocks while marshalling arugments to the error catcher into
the expected calling convention. This pushes the last block in the main
function down, so that the "entry" block may not be last BB in the
function. Instead, look up the last insertion block before generating
the catcher, and then add a call to the catcher at the end of this last
block.
Closes#2300
