[llvm] [SDAG] Use shifts if ISD::MUL is illegal when lowering ISD::CTPOP (PR #86505)

[Matt Arsenault via llvm-commits]

================
@@ -6365,7 +6365,12 @@ LegalizerHelper::lowerBitCount(MachineInstr &MI) {
 
     // Shift count result from 8 high bits to low bits.
     auto C_SizeM8 = B.buildConstant(Ty, Size - 8);
-    if (isSupported({TargetOpcode::G_MUL, {Ty, Ty}})) {
+
+    auto IsMulSupported = [this](const LLT Ty) {
+      auto Action = LI.getAction({TargetOpcode::G_MUL, {Ty, Ty}}).Action;
----------------
arsenm wrote:

Mul only has a single type index 
```suggestion
      auto Action = LI.getAction({TargetOpcode::G_MUL, {Ty}}).Action;
```

https://github.com/llvm/llvm-project/pull/86505