Smart casts for local variables not captured in a closure and not changed in a loop, see #KT-3175.

isLocalVariable added. Assignment / initialization analysis.
Control whether a variable is changed in a loop at the beginning and at the end of the loop.
Control whether a variable is captured in a closure. #KT-3175  Fixed. #KT-2266  Fixed.
Tests for variable null safety and for variables is/as operations.
Loop / closure / variable property tests are included.
Old tests changed in accordance with KT-3175. In particular, all three of testSmartcastImpossible were fixed.

compiler/testData/diagnostics/tests/BinaryCallsOnNullableValues.kt

@@ -4,7 +4,7 @@ class A() {
 
 fun f(): Unit {
   var x: Int? = <!VARIABLE_WITH_REDUNDANT_INITIALIZER!>1<!>
-  x = 1
+  x = null
   x <!UNSAFE_INFIX_CALL!>+<!> 1
   x <!UNSAFE_INFIX_CALL!>plus<!> 1
   x <!UNSAFE_INFIX_CALL!><<!> 1