[llvm] [AArch64] Remove post-decoding instruction mutations (PR #156364)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `ppc64le-lld-multistage-test` running on `ppc64le-lld-multistage-test` while building `llvm,utils` at step 12 "build-stage2-unified-tree".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/168/builds/16243

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

```
Step 12 (build-stage2-unified-tree) failure: build (failure) (timed out)
...
1086.078 [128/12/6568] Building CXX object tools/clang/unittests/CMakeFiles/AllClangUnitTests.dir/Tooling/RecursiveASTVisitorTests/CallbacksUnaryOperator.cpp.o
1086.697 [128/11/6569] Building CXX object tools/clang/unittests/CMakeFiles/AllClangUnitTests.dir/Tooling/RecursiveASTVisitorTests/CallbacksCompoundAssignOperator.cpp.o
1087.186 [128/10/6570] Building CXX object tools/clang/unittests/CMakeFiles/AllClangUnitTests.dir/Tooling/RecursiveASTVisitorTests/CallbacksCallExpr.cpp.o
1092.700 [128/9/6571] Building CXX object tools/clang/unittests/CMakeFiles/AllClangUnitTests.dir/ASTMatchers/ASTMatchersNodeTest.cpp.o
1101.521 [128/8/6572] Building CXX object tools/clang/unittests/CMakeFiles/AllClangUnitTests.dir/Tooling/RecursiveASTVisitorTests/CallbacksLeaf.cpp.o
1118.049 [128/7/6573] Building CXX object lib/Target/AMDGPU/CMakeFiles/LLVMAMDGPUCodeGen.dir/AMDGPUTargetMachine.cpp.o
1171.855 [128/6/6574] Building CXX object tools/clang/unittests/CMakeFiles/AllClangUnitTests.dir/ASTMatchers/ASTMatchersNarrowingTest.cpp.o
1273.415 [128/5/6575] Building CXX object lib/Target/AMDGPU/AsmParser/CMakeFiles/LLVMAMDGPUAsmParser.dir/AMDGPUAsmParser.cpp.o
1286.480 [128/4/6576] Building CXX object tools/clang/unittests/CMakeFiles/AllClangUnitTests.dir/AST/ASTImporterTest.cpp.o
1546.335 [128/3/6577] Building CXX object tools/clang/unittests/CMakeFiles/AllClangUnitTests.dir/ASTMatchers/ASTMatchersTraversalTest.cpp.o
command timed out: 1200 seconds without output running [b'ninja'], attempting to kill
process killed by signal 9
program finished with exit code -1
elapsedTime=2746.984648

```

</details>

https://github.com/llvm/llvm-project/pull/156364