stage2: implement array literal with explicit type

New ZIR instruction: elem_ptr_imm
This saves some memory for array literals since the element indexes are
communicated as immediate values rather than as references to other ZIR
instructions.

test/behavior/array.zig

@@ -33,8 +33,13 @@ test "array init with mult" {
     var i: [8]u8 = [2]u8{ a, 'b' } ** 4;
     try expect(std.mem.eql(u8, &i, "abababab"));
 
-    // this should cause a Value.repeated to be emitted in AIR.
-    // TODO: find a way to test that this is actually getting emmited
     var j: [4]u8 = [1]u8{'a'} ** 4;
     try expect(std.mem.eql(u8, &j, "aaaa"));
 }
+
+test "array literal with explicit type" {
+    const hex_mult: [4]u16 = .{ 4096, 256, 16, 1 };
+
+    try expect(hex_mult.len == 4);
+    try expect(hex_mult[1] == 256);
+}