stage2: implement runtime @intToEnum

* Update AIR instruction `intcast` to allow the dest type to be an
   enum.
 * LLVM backend: update `intcast` to support when the bit counts of
   operand and dest type are the same. This was already a requirement of
   the instruction previously.
 * Type: `intInfo` supports the case when the type is an enum, and
   retrieves the info for the integer tag type. This makes it pretty
   easy for backends to implement `intcast` without having to care
   explicitly that the new type is an enum. As a bonus, simple enums
   never have to go through the type system; their signedness and bit
   count are computed directly.

The "int to enum" behavior test case is now passing for stage2 in the
LLVM backend.

test/behavior/enum.zig

@@ -17,6 +17,15 @@ test "enum to int" {
     try shouldEqual(Number.Four, 4);
 }
 
+fn testIntToEnumEval(x: i32) !void {
+    try expect(@intToEnum(IntToEnumNumber, x) == IntToEnumNumber.Three);
+}
+const IntToEnumNumber = enum { Zero, One, Two, Three, Four };
+
+test "int to enum" {
+    try testIntToEnumEval(3);
+}
+
 const ValueCount1 = enum {
     I0,
 };