For user-defined increments, there were problems with Roslyn was optimizing out one of the stores.
The new transform FixRemainingIncrements now takes increments/decrements that were not detected by TransformAssignment and introduces a temporary variable that can be incremented.
This sometimes requires un-inlining via the new ILInstruction.Extract() operation.
Extract() is not supported in all possible contexts, so it is possible but unlikely that some op_Increment calls remain.
For decimals, the situation is different: legacy csc actually was optimizing "d + 1m" to "op_Increment(d)", so we can get rid of any left-over increments by undoing this optimization. This now happens in ReplaceMethodCallsWithOperators.
Daniel Grunwald