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

[Anatoly Trosinenko via llvm-commits]

atrosinenko wrote:

I wonder if there can be existing code that relies on the old behavior. Though, even if not taking the discrepancy between the documentation and the code into account, old implementation doesn't look consistent to me as it inspects **all** explicit register operands, but only checks **implicit** uses.

Furthermore, I only managed to find a single direct user of `getUsedRegs` in the existing code, the `RegAnalysis::getInstUsedRegsList` function which is documented as
```cpp
  /// Compute the set of registers \p Inst may read from, marking them in
  /// \p RegSet. If GetClobbers is true, the set set the instr may write to.
  /// Use the callgraph to fill out this info for calls.
  void getInstUsedRegsList(const MCInst &Inst, BitVector &RegSet,
                           bool GetClobbers) const;
```
and `getUsedRegs` is only called when `GetClobbers` is false:
```cpp
void RegAnalysis::getInstUsedRegsList(const MCInst &Inst, BitVector &RegSet,
                                      bool GetClobbers) const {
  if (!BC.MIB->isCall(Inst)) {
    if (GetClobbers)
      BC.MIB->getClobberedRegs(Inst, RegSet);
    else
      BC.MIB->getUsedRegs(Inst, RegSet);
    return;
  }
// ...

```

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