[llvm] [BOLT] Do not return Def-ed registers from MCPlusBuilder::getUsedRegs (PR #129890)

[Maksim Panchenko via llvm-commits]

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

Thanks! Makes sense to me.

cc: @rafaelauler - the original author of dataflow analysis.

https://github.com/llvm/llvm-project/pull/129890