[llvm] [NFC] Address bit-field storage sizes to ensure ideal packing (PR #139825)

[Aaron Ballman via llvm-commits]

================
@@ -32,8 +33,9 @@ namespace llvm {
 ///
 class BitCodeAbbrevOp {
   uint64_t Val;           // A literal value or data for an encoding.
-  bool IsLiteral : 1;     // Indicate whether this is a literal value or not.
-  unsigned Enc   : 3;     // The encoding to use.
+  LLVM_PREFERRED_TYPE(bool)
+  uint64_t IsLiteral : 1; // Indicate whether this is a literal value or not.
+  uint64_t Enc : 3;       // The encoding to use.
----------------
AaronBallman wrote:

I think this can also get a preferred type with a bit of rearranging so that the `Encoding` macro comes first.

https://github.com/llvm/llvm-project/pull/139825