implemented large parts of the 'not nil' checking

tests/reject/tuninit1.nim

@@ -0,0 +1,36 @@
+discard """
+  errormsg: "'y' might not have been initialized"
+  line:28
+"""
+
+import strutils
+
+{.warning[Uninit]:on.}
+
+proc p =
+  var x, y, z: int
+  if stdin.readLine == "true":
+    x = 34
+    
+    while false:
+      y = 999
+      break
+      
+    while true:
+      if x == 12: break
+      y = 9999
+      
+    try:
+      z = parseInt("1233")
+    except E_Base:
+      case x
+      of 34: z = 123
+      of 13: z = 34
+      else: z = 8
+  else:
+    y = 3444
+    x = 3111
+    z = 0
+  echo x, y, z
+  
+p()