[llvm] [llvm-exegesis] Add explicit support for setting DF in X86 (PR #115644)

[Clement Courbet via llvm-commits]

legrosbuffle wrote:

> It doesn't seem like the `prologepilog` pass ensures the relevant parts of `rflags` are set appropriately, at least for us.

Looking at the list of callee-saved regs in `X86CallingConv.td` we have:

```
def CSR_SysV64_RegCall_NoSSE : CalleeSavedRegs<(add RBX, RBP,
                                               (sequence "R%u", 12, 15))>;
def CSR_SysV64_RegCall       : CalleeSavedRegs<(add CSR_SysV64_RegCall_NoSSE,               
                                               (sequence "XMM%u", 8, 15))>;
```

`EFLAGS` is not in there, I'm wondering why ?

https://github.com/llvm/llvm-project/pull/115644