Before this commit we had two statements blocks for 'for' loops:
range variable declaration + iterator variable declaration + while loop.
However, BE requires a bit different loop structure to make lowerings
properly so in this commit iterator declaration & while loop were
extracted to separate block.
For-in-string loop can be generated using specialized 'length' and
'charAt' method calls, and with cached string length.
Note that update of the string variable in loop body doesn't affect
loop execution semantics.
#KT-21322 Fixed Target versions 1.2.20
Mikhail Glukhikh