Allow fewer initializer values in initializer expr lists than expected.

We now match C's behavior, where if we have an initializer list with
too-few values for the underlying type, any additional elements are
initialized to zero.

Fixes issue #123.

tests_errors/initexpr-2.ispc

@@ -1,3 +1,3 @@
-// Initializer list for array "int32[4]" must have 4 elements (has 3)
+// Initializer list for array "int32[2][4]" must have no more than 2 elements (has 3)
 
 int a[2][4] = { { 1, 2, 3 }, { 1, 2, 3, 4 }, 1 };