[llvm] [NFC][MC][Decoder] Extract fixed pieces of decoder code into new header file (PR #154802)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `ml-opt-rel-x86-64` running on `ml-opt-rel-x86-64-b1` while building `llvm` at step 5 "build-unified-tree".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/185/builds/23873

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

```
Step 5 (build-unified-tree) failure: build (failure)
...
163.825 [19/11/3911] Linking CXX executable bin/llvm-split
164.018 [19/10/3912] Linking CXX executable bin/llvm-isel-fuzzer
164.019 [19/9/3913] Linking CXX executable bin/llvm-opt-fuzzer
164.275 [19/8/3914] Linking CXX executable bin/llvm-dwp
164.340 [19/7/3915] Linking CXX executable bin/dsymutil
164.463 [19/6/3916] Linking CXX executable bin/opt
164.468 [19/5/3917] Linking CXX executable bin/llvm-lto2
164.762 [19/4/3918] Linking CXX executable bin/llvm-reduce
164.920 [19/3/3919] Linking CXX executable bin/llvm-dwarfutil
165.387 [19/2/3920] Linking CXX executable bin/llvm-lto
command timed out: 1200 seconds without output running [b'ninja', b'-j', b'64'], attempting to kill
process killed by signal 9
program finished with exit code -1
elapsedTime=1365.409432

```

</details>

https://github.com/llvm/llvm-project/pull/154802