perf: match compilation to use exfalso early

This PR lets match compilation use exfalso as soon as no alternatives
are left. This way, the compiler does not have to look at subsequent
case splits.

Author: nomeata

src/Lean/Meta/Match/Match.lean

@@ -746,12 +746,28 @@ private def checkNextPatternTypes (p : Problem) : MetaM Unit := do
           unless (← isDefEq xType eType) do
             throwError "Type mismatch in pattern: Pattern{indentExpr e}\n{← mkHasTypeButIsExpectedMsg eType xType}"
 
+def isExFalsoTransition (p : Problem) : MetaM Bool := do
+  if p.alts.isEmpty then
+    withGoalOf p do
+      let targetType ← p.mvarId.getType
+      return !targetType.isFalse
+  else
+    return false
+
+def processExFalso (p : Problem) : MetaM Problem := do
+  let mvarId' ← p.mvarId.exfalso
+  return { p with mvarId := mvarId' }
+
 private partial def process (p : Problem) : StateRefT State MetaM Unit := do
   traceState p
   let isInductive ← isCurrVarInductive p
   if isDone p then
     traceStep ("leaf")
     processLeaf p
+  else if (← isExFalsoTransition p) then
+    traceStep ("ex falso")
+    let p ← processExFalso p
+    process p
   else if hasAsPattern p then
     traceStep ("as-pattern")
     let p ← processAsPattern p