[llvm] [RISCV][DAG][TLI] Avoid scalarizing length decreasing shuffles (PR #115532)

[Matt Arsenault via llvm-commits]

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@@ -545,6 +545,11 @@ class TargetLoweringBase {
     return DefinedValues < 3;
   }
 
+  // In SDAG construction, should length decreasing shuffles be expanded
+  // to a sequence of extracts and inserts if they can't be recognized
+  // via the two extracted operand form?
+  virtual bool shouldScalarizeLengthDecreasingShuffle() const { return true; }
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arsenm wrote:

Do something unconditionally and fix it up in a combiner if preferred, like anything else. Failing that, at minimum a TLI hook needs some kind of context if it's not going to be a simple bool field 

https://github.com/llvm/llvm-project/pull/115532