[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-ppc64-aix` running on `aix-ppc64` while building `llvm` at step 6 "test-build-unified-tree-check-all".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/64/builds/1877

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

```
Step 6 (test-build-unified-tree-check-all) failure: test (failure)
******************** TEST 'lit :: googletest-timeout.py' FAILED ********************
Exit Code: 1

Command Output (stdout):
--
# RUN: at line 9
not env -u FILECHECK_OPTS "/opt/freeware/bin/python3.9" /home/powerllvm/powerllvm_env/aix-ppc64/clang-ppc64-aix/llvm-project/llvm/utils/lit/lit.py -j1 --order=lexical -v Inputs/googletest-timeout    --param gtest_filter=InfiniteLoopSubTest --timeout=1 > /home/powerllvm/powerllvm_env/aix-ppc64/clang-ppc64-aix/build/utils/lit/tests/Output/googletest-timeout.py.tmp.cmd.out
# executed command: not env -u FILECHECK_OPTS /opt/freeware/bin/python3.9 /home/powerllvm/powerllvm_env/aix-ppc64/clang-ppc64-aix/llvm-project/llvm/utils/lit/lit.py -j1 --order=lexical -v Inputs/googletest-timeout --param gtest_filter=InfiniteLoopSubTest --timeout=1
# .---command stderr------------
# | lit.py: /home/powerllvm/powerllvm_env/aix-ppc64/clang-ppc64-aix/llvm-project/llvm/utils/lit/lit/main.py:72: note: The test suite configuration requested an individual test timeout of 0 seconds but a timeout of 1 seconds was requested on the command line. Forcing timeout to be 1 seconds.
# `-----------------------------
# RUN: at line 11
FileCheck --check-prefix=CHECK-INF < /home/powerllvm/powerllvm_env/aix-ppc64/clang-ppc64-aix/build/utils/lit/tests/Output/googletest-timeout.py.tmp.cmd.out /home/powerllvm/powerllvm_env/aix-ppc64/clang-ppc64-aix/build/utils/lit/tests/googletest-timeout.py
# executed command: FileCheck --check-prefix=CHECK-INF /home/powerllvm/powerllvm_env/aix-ppc64/clang-ppc64-aix/build/utils/lit/tests/googletest-timeout.py
# RUN: at line 16
not env -u FILECHECK_OPTS "/opt/freeware/bin/python3.9" /home/powerllvm/powerllvm_env/aix-ppc64/clang-ppc64-aix/llvm-project/llvm/utils/lit/lit.py -j1 --order=lexical -v Inputs/googletest-timeout   --param gtest_filter=InfiniteLoopSubTest  --param set_timeout=1   > /home/powerllvm/powerllvm_env/aix-ppc64/clang-ppc64-aix/build/utils/lit/tests/Output/googletest-timeout.py.tmp.cfgset.out
# executed command: not env -u FILECHECK_OPTS /opt/freeware/bin/python3.9 /home/powerllvm/powerllvm_env/aix-ppc64/clang-ppc64-aix/llvm-project/llvm/utils/lit/lit.py -j1 --order=lexical -v Inputs/googletest-timeout --param gtest_filter=InfiniteLoopSubTest --param set_timeout=1
# .---command stderr------------
# | Invalid pid specified: 47513966
# | /bin/sh: kill: bad argument count
# `-----------------------------
# RUN: at line 19
FileCheck --check-prefix=CHECK-INF < /home/powerllvm/powerllvm_env/aix-ppc64/clang-ppc64-aix/build/utils/lit/tests/Output/googletest-timeout.py.tmp.cfgset.out /home/powerllvm/powerllvm_env/aix-ppc64/clang-ppc64-aix/build/utils/lit/tests/googletest-timeout.py
# executed command: FileCheck --check-prefix=CHECK-INF /home/powerllvm/powerllvm_env/aix-ppc64/clang-ppc64-aix/build/utils/lit/tests/googletest-timeout.py
# .---command stderr------------
# | /home/powerllvm/powerllvm_env/aix-ppc64/clang-ppc64-aix/build/utils/lit/tests/googletest-timeout.py:34:14: error: CHECK-INF: expected string not found in input
# | # CHECK-INF: Timed Out: 1
# |              ^
# | <stdin>:13:29: note: scanning from here
# | Reached timeout of 1 seconds
# |                             ^
# | <stdin>:37:2: note: possible intended match here
# |  Timed Out: 2 (100.00%)
# |  ^
# | 
# | Input file: <stdin>
# | Check file: /home/powerllvm/powerllvm_env/aix-ppc64/clang-ppc64-aix/build/utils/lit/tests/googletest-timeout.py
# | 
# | -dump-input=help explains the following input dump.
# | 
# | Input was:
# | <<<<<<
# |             .
# |             .
# |             .
# |             8:  
# |             9:  
# |            10: -- 
# |            11: exit: -9 
# |            12: -- 
...

```

</details>

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