Fix off-by-one error in AstTyper.

test/mjsunit/regress/regress-param-local-type.js

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 // Flags: --allow-natives-syntax
 
-function f(x) {
-  x++;
-  return x;
+// Test that we do not confuse the first local and the first parameter
+// when gathering type information.
+
+function f(a) {  // First parameter is tagged.
+  var s = '';    // First local has string type.
+  var n = 0;
+  var i = 1;
+  n = i + a;
 }
 
-f(0.5);
-f(0.5);
+f(1);
+f(1);
 %OptimizeFunctionOnNextCall(f);
-f(0.5);
+f(1);
 assertOptimized(f);