[llvm] [InstCombine] Drop NSW when converting `shl X, BW - 1` back into mul (PR #121633)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `clang-aarch64-sve2-vla-2stage` running on `linaro-g4-02` while building `llvm` at step 12 "ninja check 2".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/199/builds/643

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

```
Step 12 (ninja check 2) failure: stage 2 checked (failure)
******************** TEST 'MLIR :: Integration/Dialect/Vector/CPU/ArmSVE/sve.mlir' FAILED ********************
Exit Code: 2

Command Output (stdout):
--
# RUN: at line 1
/home/tcwg-buildbot/worker/clang-aarch64-sve2-vla-2stage/stage2/bin/mlir-opt /home/tcwg-buildbot/worker/clang-aarch64-sve2-vla-2stage/llvm/mlir/test/Integration/Dialect/Vector/CPU/ArmSVE/sve.mlir -lower-affine -convert-vector-to-scf -convert-scf-to-cf -convert-vector-to-llvm="enable-arm-sve" -finalize-memref-to-llvm -convert-func-to-llvm -convert-arith-to-llvm -convert-cf-to-llvm -canonicalize |  mlir-cpu-runner -e=entry -entry-point-result=void --march=aarch64 --mattr="+sve" -shared-libs=/home/tcwg-buildbot/worker/clang-aarch64-sve2-vla-2stage/stage2/./lib/libmlir_c_runner_utils.so |  /home/tcwg-buildbot/worker/clang-aarch64-sve2-vla-2stage/stage2/bin/FileCheck /home/tcwg-buildbot/worker/clang-aarch64-sve2-vla-2stage/llvm/mlir/test/Integration/Dialect/Vector/CPU/ArmSVE/sve.mlir
# executed command: /home/tcwg-buildbot/worker/clang-aarch64-sve2-vla-2stage/stage2/bin/mlir-opt /home/tcwg-buildbot/worker/clang-aarch64-sve2-vla-2stage/llvm/mlir/test/Integration/Dialect/Vector/CPU/ArmSVE/sve.mlir -lower-affine -convert-vector-to-scf -convert-scf-to-cf -convert-vector-to-llvm=enable-arm-sve -finalize-memref-to-llvm -convert-func-to-llvm -convert-arith-to-llvm -convert-cf-to-llvm -canonicalize
# .---command stderr------------
# | /home/tcwg-buildbot/worker/clang-aarch64-sve2-vla-2stage/llvm/mlir/test/Integration/Dialect/Vector/CPU/ArmSVE/sve.mlir:84:11: error: null operand found
# |     %lc = arith.addf %la, %lb : vector<[4]xf32>
# |           ^
# | /home/tcwg-buildbot/worker/clang-aarch64-sve2-vla-2stage/llvm/mlir/test/Integration/Dialect/Vector/CPU/ArmSVE/sve.mlir:84:11: note: see current operation: %25 = "arith.addf"(<<NULL VALUE>>, %24) <{fastmath = #arith.fastmath<none>}> : (<<NULL TYPE>>, vector<[4]xf32>) -> vector<[4]xf32>
# `-----------------------------
# error: command failed with exit status: 1
# executed command: mlir-cpu-runner -e=entry -entry-point-result=void --march=aarch64 --mattr=+sve -shared-libs=/home/tcwg-buildbot/worker/clang-aarch64-sve2-vla-2stage/stage2/./lib/libmlir_c_runner_utils.so
# .---command stderr------------
# | Error: entry point not found
# `-----------------------------
# error: command failed with exit status: 1
# executed command: /home/tcwg-buildbot/worker/clang-aarch64-sve2-vla-2stage/stage2/bin/FileCheck /home/tcwg-buildbot/worker/clang-aarch64-sve2-vla-2stage/llvm/mlir/test/Integration/Dialect/Vector/CPU/ArmSVE/sve.mlir
# .---command stderr------------
# | FileCheck error: '<stdin>' is empty.
# | FileCheck command line:  /home/tcwg-buildbot/worker/clang-aarch64-sve2-vla-2stage/stage2/bin/FileCheck /home/tcwg-buildbot/worker/clang-aarch64-sve2-vla-2stage/llvm/mlir/test/Integration/Dialect/Vector/CPU/ArmSVE/sve.mlir
# `-----------------------------
# error: command failed with exit status: 2

--

********************


```

</details>

https://github.com/llvm/llvm-project/pull/121633