[llvm] [BOLT][BTI] Add MCPlusBuilder::insertBTI (PR #167329)

[Peter Waller via llvm-commits]

https://github.com/peterwaller-arm commented:

The assertion needs fixing before this can land, but otherwise LGTM - will give approval once fixed. Also made some other minor suggestions.

https://github.com/llvm/llvm-project/pull/167329