[llvm] [TableGen] Add support for DefaultMode in per-HwMode encode/decode. (PR #83029)

[Jason Eckhardt via llvm-commits]

nvjle wrote:

I hope one of @topperc @jyknight @jmolloy @0x59616e @wangpc-pp (based on recent commits or past reviews to the same general area) might review this small patch, thank you.

https://github.com/llvm/llvm-project/pull/83029