Extra optimizations

[pvasilek]

Draft proposal for extra decompiler optimizations: universe case, dead IFs.

[pvasilek]

Replaces
switch (...) {
case [long.MinValue..long.MaxValue]: { CODE; }
}
with
CODE;

Also, draft for
if (...) nop; [else nop;] -> nop;
if (...) { x; } x; -> x;
if (...) { x; } else { x; } -> x;
But incomplete, need someones review and fix.

[dgrunwald]

ILInstruction.ToString() is only meant for debug output and not for comparisons.

[dgrunwald]

The decompiler isn't supposed to optimize the code. If the compiler is inefficient and generates redundant conditions, we don't want to hide those from the user.

I would be OK with these changes behind the "Remove dead code" option, but not in the default mode.

[pvasilek]

I think you are right and this one may be rejected to become a part of de4dot. Plain decompiler is useless against highly obfuscated assemblies.

[dgrunwald]

To handle obfuscated code we'd first need to fix #901 by making our bytecode importer (ILReader) more like the logic in the .NET runtime.
Unreachable code needs to be discarded before we analyze the stack types; so ControlFlowSimplification is too late for that.

More advanced de-obfuscation, unpacking, etc. are a non-goal for ILSpy. There's already de4dot for that.