[llvm] [ConstantRange] Allow casting to the same bitwidth. NFC (PR #170102)

[Nikita Popov via llvm-commits]

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

LGTM. We changed APInt to allow this some time ago, makes sense to also do this in ConstantRange.

https://github.com/llvm/llvm-project/pull/170102