[clang] [llvm] [X86] Support branch hint (PR #97721)

[Shengchen Kan via cfe-commits]

================
@@ -749,6 +749,11 @@ def TuningUseGLMDivSqrtCosts
     : SubtargetFeature<"use-glm-div-sqrt-costs", "UseGLMDivSqrtCosts", "true",
         "Use Goldmont specific floating point div/sqrt costs">;
 
+// Starting with Redwood Cove architecture, the branch has branch taken hint
+// (i.e., instruction prefix 3EH).
+def TuningBranchHint: SubtargetFeature<"branch-hint", "HasBranchHint", "true",
----------------
KanRobert wrote:

See Intel SDM
```
— Branch hints1:
• 2EH—Branch not taken (used only with Jcc instructions).
• 3EH—Branch taken (used only with Jcc instructions).

1. Some earlier microarchitectures used these as branch hints, but recent generations have not and they are reserved for future hint usage.
```

https://github.com/llvm/llvm-project/pull/97721