Reduce DataLang cutsets by rematerialising constants closer to use

[tanyongkiam]

Reduce DataLang cutsets by rematerialising constants closer to use

This follow-up was originally reported in a comment on #1042. The original
problem in #1042 has been fixed, but this is a separate remaining opportunity.

bvi_to_data can compute cheap constant arguments early, leaving them live
across later operations which carry cutsets. For example, this BVI tail call:

(func parse_vb_string@3246 (a)
   (call parse_vb_string_aux@3240 (op (Cons 0))
      (op (Const 0)) (op (Const 1))
      (let
         (b <- (op Add (op LengthByte (var a)) (op (Const ~1))))
         (if (op Less (op (Const 0)) (var b)) (op (Const 0)) (var b)))
      (op (Const 0)) (var a)))

was lowered to DataLang in this shape:

(func parse_vb_string@3246 (0)
   (seq
      (1 := (Cons 0) () none)
      (2 := (Const 0) () none)
      (3 := (Const 1) () none)
      (4 := LengthByte (0) none)
      (5 := (Const ~1) () none)
      (6 := Add (5 4) (some {0,1,2,3,4,5}))
      (7 := (Const 0) () none)
      (8 := Less (6 7) (some {0,1,2,3,6,7}))
      (if 8
         (seq
            (9 := (Const 0) () none)
            (10 := 9))
         (10 := 6))
      (11 := (Const 0) () none)
      (call none parse_vb_string_aux@3240 (1 2 3 10 11 0) none)))

The constants in variables 1, 2, and 3 are created before the
LengthByte, Add, and Less computations, so they unnecessarily remain in
the intervening cutsets even though they are cheap to construct immediately
before the final call.

One approach discussed with @myreen was constant propagation/rematerialisation
before data_live. Another related opportunity is for bvi_to_data to compute
arguments which induce cutsets before cheap arguments that can safely be
computed later. Any reordering must, of course, preserve the language's
evaluation-order, exception, and effect requirements.

The expected result for examples of this form is that cheap constants are
materialised near their eventual use and do not enlarge unrelated intervening
cutsets.

Original observation and example by @tanyongkiam; issue text organized by Codex (OpenAI).