[llvm] TargetInstrInfo: squelch a signedness warning on MSVC (PR #74078)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

This will help, but there are a LOT of these warnings in other files - I don't have the time to address these right now. Ideally you'd be able to find a buildbot that lists them in the build stdout (and not just the <optional> header line)

https://github.com/llvm/llvm-project/pull/74078