[llvm] [TTI] getScalingFactorCost should return InstructionCost::getInvalid() instead of -1. (PR #129802)

[Craig Topper via llvm-commits]

================
@@ -364,7 +364,7 @@ class TargetTransformInfoImplBase {
                               Scale, AddrSpace, /*I=*/nullptr,
                               BaseOffset.getScalable()))
       return 0;
-    return -1;
+    return InstructionCost::getInvalid();
----------------
topperc wrote:

The only caller I've found so far is in `getScalingFactorCost` in LSR. If `isAMCompletelyFolded` returns true for an `LSRUse::Address` kind, `TTI.getScalingFactorCost` is called twice. Right after the calls there is an assert that the cost is valid. That used to be an assert that the cost is positive. You can see this on line 1782 of the diff to LoopStrengthReduce.cpp in https://github.com/llvm/llvm-project/commit/43ace8b5ce07a90b3562bbd0488f88e6290627c4

I think there is an implicit contract that if `isAMCompletelyFolded` returns true, then `getScalingFactorCost` should return a valid cost.

https://github.com/llvm/llvm-project/pull/129802