[llvm] [InstCombine] Handle scalable splats of constants in getMinimumFPType (PR #132960)

[David Sherwood via llvm-commits]

================
@@ -1685,11 +1685,12 @@ static Type *getMinimumFPType(Value *V, bool PreferBFloat) {
       return T;
 
   // We can only correctly find a minimum type for a scalable vector when it is
-  // a splat. For splats of constant values the fpext is wrapped up as a
-  // ConstantExpr.
-  if (auto *FPCExt = dyn_cast<ConstantExpr>(V))
-    if (FPCExt->getOpcode() == Instruction::FPExt)
-      return FPCExt->getOperand(0)->getType();
+  // a splat.
+  if (auto *FPCE = dyn_cast<ConstantExpr>(V))
+    if (isa<ScalableVectorType>(V->getType()))
+      if (auto *Splat = dyn_cast<ConstantFP>(FPCE->getSplatValue()))
----------------
david-arm wrote:

In `InstCombinerImpl::visitCallInst` there is a similar example of using splats:

```
    // Handle mul by one:
    if (Constant *CV1 = dyn_cast<Constant>(Arg1))
      if (ConstantInt *Splat =
              dyn_cast_or_null<ConstantInt>(CV1->getSplatValue()))
```

and I wondered if it was worth doing the same here. Perhaps I'm wrong, but I thought splats aren't restricted to scalable vectors I think and would apply to fixed-width too? For example,

```
  if (auto *CFP = dyn_cast<Constant>(V))
    if (auto *Splat = dyn_cast_or_null<ConstantFP>(CFP->getSplatValue()))
```

https://github.com/llvm/llvm-project/pull/132960