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

[Anatoly Trosinenko via llvm-commits]

atrosinenko wrote:

Ping.

Rebased the PR onto current `main` branch. To simplify the review, the patch is comprised of two commits:
* The first one updates `getUsedRegs` and adds corresponding unit tests. Along the test cases, it adds two `assertRegMask` helper functions which generalize the idea of `testRegAliases` and add a bit of `std::initializer_list` syntactic sugar.
* The second commit makes use of `assertRegMask` in the original `testRegAliases` function and updates the rest of the test cases to use the same style of passing their register lists.

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