[llvm] [GISel][CombinerHelper] Combine op(trunc(x), trunc(y)) -> trunc(op(x, y)) (PR #89023)

[Matt Arsenault via llvm-commits]

================
@@ -3083,8 +3083,10 @@ bool CombinerHelper::matchHoistLogicOpWithSameOpcodeHands(
     return false;
   case TargetOpcode::G_ANYEXT:
   case TargetOpcode::G_SEXT:
-  case TargetOpcode::G_ZEXT: {
+  case TargetOpcode::G_ZEXT:
+  case TargetOpcode::G_TRUNC: {
     // Match: logic (ext X), (ext Y) --> ext (logic X, Y)
+    // Match: logic (trunc X), (trunc Y) -> trunc (logic X, Y)
----------------
arsenm wrote:

The logic for the DAG combine treats the extend and truncate cases a bit differently. The truncate case considers whether the extend and truncate are free, and doesn't do this if they are. Arguably this could be handled by the target not adding the combine, but I don't think we have a way to add combines for specific types right now 

https://github.com/llvm/llvm-project/pull/89023