[llvm] [DAG] Change `isExtractSubvectorCheap` into `getExtractSubvectorCost` (PR #213614)

[Simon Pilgrim via llvm-commits]

================
@@ -3540,13 +3540,22 @@ class LLVM_ABI TargetLoweringBase {
     return false;
   }
 
-  /// Return true if EXTRACT_SUBVECTOR is cheap for extracting this result type
-  /// from this source type with this index. This is needed because
-  /// EXTRACT_SUBVECTOR usually has custom lowering that depends on the index of
-  /// the first element, and only the target knows which lowering is cheap.
-  virtual bool isExtractSubvectorCheap(EVT ResVT, EVT SrcVT,
-                                       unsigned Index) const {
-    return false;
+  /// Return the cost of extracting a subvector of type \p ResVT from a vector
+  /// of type \p SrcVT, starting at element \p Index:
+  ///
+  ///   0 - Free: lowers to no instruction at all, e.g. a subregister copy.
+  ///   1 - Cheap: lowers to at most one instruction, and may still be free if
+  ///       the target can fold the extract into the instruction consuming it
+  ///       (e.g. a widening op that reads the high half of a register).
+  ///   2 - Expensive: needs a shuffle sequence that cannot be folded away.
----------------
RKSimon wrote:

I would give real money for TCC_Expensive to be removed, let alone leak into DAG - adding a cost mechanism encourages complex accumulation and comparisons, which doesn't usually remain stable in the DAG (NegatibleCost is the only one that sort of works and that's because NegatedExpression uses target-specific overrides everywhere).

Have you looked at moving this in the other direction and replacing isExtractSubvectorCheap with a more limited isExtractSubvectorFree?

https://github.com/llvm/llvm-project/pull/213614