separate if statements for Incr and Decr operators

[Panos90]

The condition on StatementFor.cpp:103 eliminates any chance of an overflow/underflow when CGOptions::fast_execution() is disabled.

However, in StatementFor.cpp:113, the fact that incr_op is assigned depending only on the condition limit >= init re-introduces the chance of an overflow/underflow when limit == init.

I 'm not sure if this behaviour is intentional. If it is you can ignore this pull request.

[jxyang]

The original intention was to consolidate all ++/-- cases in one if-statement. The code has been changed much so that a separation is warranted. Nice catch.