stage2: LLVM backend: fix const packed structs

When doing LLVM const bit shifting we must make sure the integer bit
sizes are wide enough or else LLVM gives us a poison result.

test/behavior/enum_llvm.zig

@@ -47,3 +47,46 @@ test "enum literal casting to optional" {
 
     try expect(bar.? == Bar.B);
 }
+
+const A = enum(u3) { One, Two, Three, Four, One2, Two2, Three2, Four2 };
+const B = enum(u3) { One3, Two3, Three3, Four3, One23, Two23, Three23, Four23 };
+const C = enum(u2) { One4, Two4, Three4, Four4 };
+
+const BitFieldOfEnums = packed struct {
+    a: A,
+    b: B,
+    c: C,
+};
+
+const bit_field_1 = BitFieldOfEnums{
+    .a = A.Two,
+    .b = B.Three3,
+    .c = C.Four4,
+};
+
+test "bit field access with enum fields" {
+    var data = bit_field_1;
+    try expect(getA(&data) == A.Two);
+    try expect(getB(&data) == B.Three3);
+    try expect(getC(&data) == C.Four4);
+    comptime try expect(@sizeOf(BitFieldOfEnums) == 1);
+
+    data.b = B.Four3;
+    try expect(data.b == B.Four3);
+
+    data.a = A.Three;
+    try expect(data.a == A.Three);
+    try expect(data.b == B.Four3);
+}
+
+fn getA(data: *const BitFieldOfEnums) A {
+    return data.a;
+}
+
+fn getB(data: *const BitFieldOfEnums) B {
+    return data.b;
+}
+
+fn getC(data: *const BitFieldOfEnums) C {
+    return data.c;
+}