[llvm] [Support][APint] Optimize fallback case in APInt::reverseBits (PR #189291)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `ppc64le-mlir-rhel-clang` running on `ppc64le-mlir-rhel-test` while building `llvm` at step 6 "test-build-check-mlir-build-only-check-mlir".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/129/builds/41765

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

```
Step 6 (test-build-check-mlir-build-only-check-mlir) failure: 1200 seconds without output running [b'ninja', b'check-mlir'], attempting to kill
...
PASS: MLIR-Unit :: IR/./MLIRIRTests/104/145 (3996 of 4007)
PASS: MLIR-Unit :: Interfaces/./MLIRInterfacesTests/12/30 (3997 of 4007)
PASS: MLIR :: mlir-tblgen/cpp-class-comments.td (3998 of 4007)
PASS: MLIR :: mlir-runner/simple.mlir (3999 of 4007)
PASS: MLIR-Unit :: IR/./MLIRIRTests/0/145 (4000 of 4007)
PASS: MLIR-Unit :: Interfaces/./MLIRInterfacesTests/13/30 (4001 of 4007)
PASS: MLIR-Unit :: Pass/./MLIRPassTests/11/14 (4002 of 4007)
PASS: MLIR-Unit :: Interfaces/./MLIRInterfacesTests/11/30 (4003 of 4007)
PASS: MLIR-Unit :: IR/./MLIRIRTests/37/145 (4004 of 4007)
PASS: MLIR-Unit :: IR/./MLIRIRTests/38/145 (4005 of 4007)
command timed out: 1200 seconds without output running [b'ninja', b'check-mlir'], attempting to kill
process killed by signal 9
program finished with exit code -1
elapsedTime=2331.523152

```

</details>

https://github.com/llvm/llvm-project/pull/189291