Improve complex pattern match

[bobzhang]

OCaml's pattern match compiler generates very efficient code, however, it is not tweaked for code size,
see such example:

let f x =
   let y  = match x with a0::a1::a2::a3::a4::a5::a6::_ -> a0+a1+a2+a3+a4+a5+a6 | _ -> 0 in y  + 2 ;;

The common pattern is that for each pattern, we generate lots of temporay let match/id =a xx in if match/id

if we lift the definition to the toplevel.

var match/id
if (match/id = xx) 

Then we can simplify further

var match/id0, match/id1
if(match/id0 = xx0 && match/id1 = xx1){
   ..
} else {
 ...
}

[bobzhang]

suppose we introduce aif inside IR, it would be

aif (match/1086 = (field  1 x/1009))
     aif (match/1087 = (field 1 match/1086))
        aif (match/1088 = (field 1 match/1087))
           aif (match/1089 = (field 1 match/1088)
              (field 0 match/1086 )+ (field 1 match/1087) + (field 2 match/1088) + (field 3 match/1089)
              0
       0
   0
0          

could be simplified as

aif (match/1086 = (field  1 x/1009)
      match/1087 = (field 1 match/1086)
      match/1088 = (field 1 match/1087)
      match/1089 = (field 1 match/1088))
    (field 0 match/1086 )+ (field 1 match/1087) + (field 2 match/1088) + (field 3 match/1089)
0

[stale]

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