[llvm] [InstCombine] Add missing constant check (PR #170068)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

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

Full details are available at: https://lab.llvm.org/buildbot/#/builders/157/builds/42648

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

```
Step 6 (test-build-unified-tree-check-flang) failure: 1200 seconds without output running [b'ninja', b'check-flang'], attempting to kill
...
XFAIL: Flang :: Semantics/unlockstmt02.f90 (4020 of 4031)
PASS: Flang :: Semantics/unlockstmt01.f90 (4021 of 4031)
PASS: Flang :: Transforms/external-name-interop-symref-array.fir (4022 of 4031)
PASS: Flang :: Semantics/widening.f90 (4023 of 4031)
PASS: Flang :: Transforms/debug-line-table.fir (4024 of 4031)
PASS: Flang :: Transforms/stack-arrays.f90 (4025 of 4031)
PASS: Flang :: Semantics/undef-result01.f90 (4026 of 4031)
PASS: Flang :: Transforms/OpenACC/acc-implicit-data-fortran.F90 (4027 of 4031)
PASS: Flang :: Transforms/debug-dwarf-version.fir (4028 of 4031)
PASS: Flang :: Driver/linker-options.f90 (4029 of 4031)
command timed out: 1200 seconds without output running [b'ninja', b'check-flang'], attempting to kill
process killed by signal 9
program finished with exit code -1
elapsedTime=3556.691538

```

</details>

https://github.com/llvm/llvm-project/pull/170068