Simplify case of and pattern matching implementation

[JaroslavTulach]

Pavel Marek reports:

Originally posted in #10507

• Convert rest of global typing pass group to mini passes #11717 will be blocked until I fully understand pattern matching IR and runtime interpretation.
• The whole NestedPatternMatch IR pass seems unnecessary and too complicated - creates a lot of IR nodes.
• Before migrating this pass to a mini pass, we could try to simplify the whole pattern matching IR (both Scala and Truffle).

• attempt to turn if/then/else into a construct and not a function
• e.g. Recognize if-like Tree.MultiSegmentApp as IfThenElse IR #11365 is related and shows problems one will be facing

The handling of case of pattern and its BranchXyzNodes were clearly created before Truffle was the target platform. As such

• too much work is being done in IR
• while most of the work should be done in Truffle nodes

Goals

• avoid creation of temporary variables in IR
• consume IR before/without NestedPatternMatch in IrToTruffle
  • what's missing
  • are the problems the same as Recognize if-like Tree.MultiSegmentApp as IfThenElse IR #11365?
  • would the solution also help IfThenElse IR to avoid 3x slower IfVsCaseBenchmarks_ifBench6In #9165?
• describe how NestedPatternMatch will be simplified/removed