Allow `let` just before pure/return in ApplicativeDo

The following is currently rejected:

```haskell
-- F is an Applicative but not a Monad
x :: F (Int, Int)
x = do
  a <- pure 0
  let b = 1
  pure (a, b)
```

This has bitten me multiple times. This MR contains a simple fix:
only allow a "let only" segment to be merged with the next (and not
the previous) segment. As a result, when the last one or more
statements before pure/return are `LetStmt`s, there will be one
more segment containing only those `LetStmt`s.

Note that if the `let` statement mentions a name bound previously, then
the program is still rejected, for example

```haskell
x = do
  a <- pure 0
  let b = a + 1
  pure (a, b)
```

or the example in #18559. To support this would require a more
complex approach, but this is IME much less common than the
previous case.

1 files changed, 2 insertions, 2 deletions

diff --git a/testsuite/tests/ado/ado001.stdout b/testsuite/tests/ado/ado001.stdout
index 365860f55e..f4f576e084 100644
--- a/testsuite/tests/ado/ado001.stdout
+++ b/testsuite/tests/ado/ado001.stdout
@@ -6,7 +6,7 @@ a; g
 a; (b | (c; (d; (e | (f; g)))))
 (a | b); (c | d)
 a; ((b | c) | d)
-((a | (b; c)) | d) | e
+(((a | (b; c)) | d) | e) | pure
 ((a | b); (c | d)) | e
-a | b
+(a | b) | pure
 a | (b; c)