'while' and 'do-while' loop generator fixes.

Generate 'do-while' loop body as IrComposite, because variables declared
in loop body should be visible in loop condition.
Wrap 'do-while' loop in IrBlock so that variables declared in loop body
are not visible outside of the loop.

Generate 'while' and 'do-while' loops as expressions of type Unit.

compiler/testData/ir/irText/expressions/badBreakContinue.txt

@@ -7,7 +7,7 @@ FILE /badBreakContinue.kt
     BLOCK_BODY
       WHILE label=L1 origin=WHILE_LOOP
         condition: CONST Boolean type=kotlin.Boolean value='true'
-        body: BLOCK type=kotlin.Nothing origin=null
+        body: BLOCK type=kotlin.Unit origin=null
           ERROR_EXPR 'Loop not found for break expression: break@ERROR' type=kotlin.Nothing
           ERROR_EXPR 'Loop not found for continue expression: continue@ERROR' type=kotlin.Nothing
   FUN public fun test3(): kotlin.Unit