[llvm] [NFCI][MC][DecoderEmitter] Fix BitWidth for fixed length inst encodings (PR #154934)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `lldb-arm-ubuntu` running on `linaro-lldb-arm-ubuntu` while building `llvm` at step 6 "test".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/18/builds/20644

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

```
Step 6 (test) failure: build (failure)
...
PASS: lldb-unit :: Core/./LLDBCoreTests/93/470 (2429 of 3633)
PASS: lldb-unit :: Core/./LLDBCoreTests/94/470 (2430 of 3633)
PASS: lldb-unit :: Core/./LLDBCoreTests/97/470 (2431 of 3633)
PASS: lldb-unit :: Core/./LLDBCoreTests/96/470 (2432 of 3633)
PASS: lldb-unit :: Core/./LLDBCoreTests/95/470 (2433 of 3633)
PASS: lldb-unit :: Core/./LLDBCoreTests/98/470 (2434 of 3633)
PASS: lldb-unit :: Core/./LLDBCoreTests/99/470 (2435 of 3633)
PASS: lldb-unit :: DAP/./DAPTests/1/74 (2436 of 3633)
PASS: lldb-unit :: DAP/./DAPTests/0/74 (2437 of 3633)
PASS: lldb-unit :: DAP/./DAPTests/11/74 (2438 of 3633)
FAIL: lldb-unit :: DAP/./DAPTests/10/74 (2439 of 3633)
******************** TEST 'lldb-unit :: DAP/./DAPTests/10/74' FAILED ********************
Script(shard):
--
GTEST_OUTPUT=json:/home/tcwg-buildbot/worker/lldb-arm-ubuntu/build/tools/lldb/unittests/DAP/./DAPTests-lldb-unit-1998537-10-74.json GTEST_SHUFFLE=0 GTEST_TOTAL_SHARDS=74 GTEST_SHARD_INDEX=10 /home/tcwg-buildbot/worker/lldb-arm-ubuntu/build/tools/lldb/unittests/DAP/./DAPTests
--

Script:
--
/home/tcwg-buildbot/worker/lldb-arm-ubuntu/build/tools/lldb/unittests/DAP/./DAPTests --gtest_filter=DisconnectRequestHandlerTest.DisconnectTriggersTerminateCommands
--
../llvm-project/lldb/unittests/DAP/Handler/DisconnectTest.cpp:51: Failure
Actual function call count doesn't match EXPECT_CALL(client, Received(Output("1\n")))...
         Expected: to be called once
           Actual: never called - unsatisfied and active

../llvm-project/lldb/unittests/DAP/Handler/DisconnectTest.cpp:52: Failure
Actual function call count doesn't match EXPECT_CALL(client, Received(Output("2\n")))...
         Expected: to be called twice
           Actual: called once - unsatisfied and active


../llvm-project/lldb/unittests/DAP/Handler/DisconnectTest.cpp:51
Actual function call count doesn't match EXPECT_CALL(client, Received(Output("1\n")))...
         Expected: to be called once
           Actual: never called - unsatisfied and active

../llvm-project/lldb/unittests/DAP/Handler/DisconnectTest.cpp:52
Actual function call count doesn't match EXPECT_CALL(client, Received(Output("2\n")))...
         Expected: to be called twice
           Actual: called once - unsatisfied and active



********************
PASS: lldb-unit :: DAP/./DAPTests/12/74 (2440 of 3633)
PASS: lldb-unit :: DAP/./DAPTests/13/74 (2441 of 3633)
PASS: lldb-unit :: DAP/./DAPTests/15/74 (2442 of 3633)
PASS: lldb-unit :: DAP/./DAPTests/16/74 (2443 of 3633)

```

</details>

https://github.com/llvm/llvm-project/pull/154934