[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-devrel-x86-64` running on `ml-opt-devrel-x86-64-b1` while building `llvm` at step 5 "build-unified-tree".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/175/builds/23880

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

```
Step 5 (build-unified-tree) failure: build (failure)
...
163.787 [21/9/3911] Linking CXX executable bin/llc
164.083 [21/8/3912] Linking CXX executable bin/llvm-split
164.139 [21/7/3913] Linking CXX executable bin/opt
164.346 [21/6/3914] Linking CXX executable bin/bugpoint
164.360 [20/6/3915] Building CXX object tools/bugpoint-passes/CMakeFiles/BugpointPasses.dir/TestPasses.cpp.o
164.385 [19/6/3916] Linking CXX shared module lib/BugpointPasses.so
164.398 [19/5/3917] Linking CXX executable bin/llvm-lto2
165.081 [19/4/3918] Linking CXX executable bin/llvm-reduce
165.167 [19/3/3919] Linking CXX executable bin/llvm-opt-fuzzer
165.381 [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=1365.439103

```

</details>

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