[llvm] [TableGen][DecoderEmitter] Fix broken AdditionalEncoding support (PR #155057)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `lldb-remote-linux-win` running on `as-builder-10` while building `llvm` at step 16 "test-check-lldb-unit".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/197/builds/8315

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 16 (test-check-lldb-unit) failure: 1200 seconds without output running [b'cmake', b'--build', b'.', b'--target', b'check-lldb-unit'], attempting to kill
...
PASS: lldb-unit :: ValueObject/./LLDBValueObjectTests.exe/4/12 (942 of 952)
PASS: lldb-unit :: ValueObject/./LLDBValueObjectTests.exe/3/12 (943 of 952)
PASS: lldb-unit :: ValueObject/./LLDBValueObjectTests.exe/5/12 (944 of 952)
PASS: lldb-unit :: ValueObject/./LLDBValueObjectTests.exe/7/12 (945 of 952)
PASS: lldb-unit :: ValueObject/./LLDBValueObjectTests.exe/6/12 (946 of 952)
PASS: lldb-unit :: ValueObject/./LLDBValueObjectTests.exe/8/12 (947 of 952)
PASS: lldb-unit :: ValueObject/./LLDBValueObjectTests.exe/9/12 (948 of 952)
PASS: lldb-unit :: tools/lldb-server/tests/./LLDBServerTests.exe/0/1 (949 of 952)
PASS: lldb-unit :: Target/./TargetTests.exe/52/54 (950 of 952)
PASS: lldb-unit :: Process/gdb-remote/./ProcessGdbRemoteTests.exe/8/35 (951 of 952)
command timed out: 1200 seconds without output running [b'cmake', b'--build', b'.', b'--target', b'check-lldb-unit'], attempting to kill
program finished with exit code 1
elapsedTime=1247.331317

```

</details>

https://github.com/llvm/llvm-project/pull/155057