Add functions to find the matching ELSE, ENDIF expressions

ExpressionId FindMatchingEndif(ExpressionId id)
   - Finds the closing ENDIF expression for a given IF expression.

ExpressionId FindMatchingElse(ExpressionId id)
   - Finds the ELSE expression for a given IF expression.
   - Returns -1, if this IF doesn't have an else

This patch is a preparation to the code analyzing required
for various optimization passes.

Change-Id: I893102a757c6a0bcacbcc1190c2b8ef08314eb97