Fix scala#2903: Reduce the depth of trees generated in PatternMatcher

* Extract all match arguments before checking conditions on them like scalac does.
  This avoids an extra nested block for each match variable.
* Merge conditions of nested `if` expressions if their `else` branch is the same.
  This optimization combined with the previous removes most of the nested `if`s
  created to check the matched args.

Author: nicolasstucki

compiler/src/dotty/tools/dotc/transform/PatternMatcher.scala

@@ -846,11 +846,11 @@ object PatternMatcher {
             /** Merge nested `if`s that have the same `else` branch into a single `if`.
              *  This optimization targets calls to label defs for case failure jumps to next case.
              */
-            def emmitWithMashedConditions(plans: List[TestPlan]): Tree = {
+            def emitWithMashedConditions(plans: List[TestPlan]): Tree = {
               val plan = plans.head
               plan.onSuccess match {
                 case plan2: TestPlan if plan.onFailure == plan2.onFailure =>
-                  emmitWithMashedConditions(plan2 :: plans)
+                  emitWithMashedConditions(plan2 :: plans)
                 case _ =>
                   def emitCondWithPos(plan: TestPlan) = emitCondition(plan).withPos(plan.pos)
                   val conditions =
@@ -862,7 +862,7 @@ object PatternMatcher {
               }
 
             }
-            emmitWithMashedConditions(plan :: Nil)
+            emitWithMashedConditions(plan :: Nil)
           }
         case LetPlan(sym, body) =>
           seq(ValDef(sym, initializer(sym).ensureConforms(sym.info)) :: Nil, emit(body))