Sorry for the big PR. The bytecode fronted has a lot of shared state that makes it difficult to change in small steps.

Moved the logic for assigning the values of operands to stack locals out of OperandStack and into Operand.
That means that the assignments are created when the operands are used as a local/immediate with the toLocal and toImmediate methods instead of when they are popped of the stack with popLocal and popImmediate.
This fixes some issues where the wrong pop method was used which caused unnecessary stack local variables to get created.
This also results in improved type-safety and gets rid of a bunch of casts, because toLocal and toImmediate actually return the Local and Immediate types instead of a generic Operand.

Significantly simplified merging logic in StackFrame.

All convert methods had a code path for the first time they were called and for merging subsequent calls.
These two code paths have been combined now.

Factored a bunch of duplicated code into the new changeStackLocal in Operand.
The old version actually incorrectly merged operands at times (See SwitchExprWithYieldTest).

This code

int k = 1;
String s = "";

switch (k) {
  case 1:
    s += "single";
  default: // note the fall-through
    s += "somethingElse";
};

would previously produce

$l1 = 1
$l2 = ""
$stack8 = $l1
switch($stack8)
case 1: goto label13
default: goto label14
label13:
$l2 = dynamicinvoke "makeConcatWithConstants" <java.lang.String (java.lang.String)>($l2) <java.lang.invoke.StringConcatFactory: java.lang.invoke.CallSite makeConcatWithConstants(java.lang.invoke.MethodHandles$Lookup,java.lang.String,java.lang.invoke.MethodType,java.lang.String,java.lang.Object[])>("\u0001single")
label14:
$stack4 = $l2
$stack5 = dynamicinvoke "makeConcatWithConstants" <java.lang.String (java.lang.String)>($l2) <java.lang.invoke.StringConcatFactory: java.lang.invoke.CallSite makeConcatWithConstants(java.lang.invoke.MethodHandles$Lookup,java.lang.String,java.lang.invoke.MethodType,java.lang.String,java.lang.Object[])>("\u0001somethingElse")
// there is a bug here: `$l2 = $stack5` is missing

but now produces

$l1 = 1
$l2 = ""
switch($l1)
case 1: goto label13
default: goto label14
label13:
$l2 = dynamicinvoke \"makeConcatWithConstants\" <java.lang.String (java.lang.String)>($l2) <java.lang.invoke.StringConcatFactory: java.lang.invoke.CallSite makeConcatWithConstants(java.lang.invoke.MethodHandles$Lookup,java.lang.String,java.lang.invoke.MethodType,java.lang.String,java.lang.Object[])>(\"\\u0001single\")
label14:
$l2 = dynamicinvoke \"makeConcatWithConstants\" <java.lang.String (java.lang.String)>($l2) <java.lang.invoke.StringConcatFactory: java.lang.invoke.CallSite makeConcatWithConstants(java.lang.invoke.MethodHandles$Lookup,java.lang.String,java.lang.invoke.MethodType,java.lang.String,java.lang.Object[])>(\"\\u0001somethingElse\")

This adds the missing assignment and doesn't produce so many stack local variables.

Fixed updating usages of the value/stackLocal in Operand. This is now part of the changeStackLocal method.

Simplified duplication instructions (Thanks to Operand.DWORD_DUMMY).

Removed some commented out code.

Fixed an issue in the ReplaceUseStmtVisitor that would produce stale results.

Fixes #698
Fixes #631

TODO:

• rebase/merge to newest develop
• fix tests
• add comments/documentation on how the bytecode frontend works for the next person that looks at this code
• don't create stack locals when a normal local could be used
• rename/rework StackFrame (it has nothing to do with stack frames)
• remove duplicated logic in each instruction conversion method for creating and merging the instruction
• don't merge operands when they all have the same value
• find out what addReadOperandAssignments_internal does and maybe remove it
• improve statement position info for operands and statements