[llvm] [LV] Convert gather loads with invariant stride into strided loads (PR #147297)

[Mel Chen via llvm-commits]

Mel-Chen wrote:

@fhahn 490b3e7047bcce5720424f9326d33fc25d5da354
I tried adding VPWidenMemIntrinsicRecipe. In principle, it should inherit from VPWidenIntrinsicRecipe and reuse most of its code. However, I noticed that the inheritance chain of VPWidenIntrinsicRecipe is:
```
VPSingleDefRecipe
  └── VPRecipeWithIRFlags 
        └── VPWidenIntrinsicRecipe
```
Since we will need to support strided stores in the future, VPWidenMemIntrinsicRecipe may not define any VPValue. Can we still let VPWidenMemIntrinsicRecipe directly inherit from VPWidenIntrinsicRecipe in this case?

https://github.com/llvm/llvm-project/pull/147297