[llvm] Select immediate binop optimization (PR #212163)

[Keith Dunwoody via llvm-commits]

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@@ -3064,6 +3064,14 @@ class LLVM_ABI TargetLoweringBase {
     return Value == 0;
   }
 
+  /// Return true if the specified immediate is legal immediate operation,
+  /// that is the target has instructions which can perform the operation
+  /// with the immediate without having to materialize the immediate into a
+  /// register.
+  virtual bool isLegalImmediate(unsigned Opc, const SDValue Val) const {
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keithdunwoody wrote:

@arsenm If there's an existing target hook for this, I couldn't find it.

The closest I found was [isLegalAddImmediate](https://llvm.org/doxygen/classllvm_1_1TargetLoweringBase.html#a8ccde50eadcc47f62c4076681b969864).  And I could resolve the bug I was encountering narrowly just by applying this to `add`.  But the same pattern will also apply for other binary operations which allow immediates.  So I was trying to follow the pattern of that function but extend to other operations.

Can you suggest a better hook?  I'd also considered `isLegalBinOpImmediate()` (same signature) but decided that there's no intrinsic reason such a function should only apply to binary ops.

https://github.com/llvm/llvm-project/pull/212163