[llvm] [ARM] Fix undefined behavior in `isLegalAddImmediate` (PR #132219)

[Oliver Stannard via llvm-commits]

https://github.com/ostannard approved this pull request.

LGTM, I'm also surprised that this doesn't exist in either C++ or MathExtras.h.

https://github.com/llvm/llvm-project/pull/132219