[llvm] [Matrix] Use DenseMap for ShapeMap instead of ValueMap. (PR #118282)

[Adam Nemet via llvm-commits]

anemet wrote:

> > Certainly the right direction but can you explain how do we end up modifying the entry in ShapeMap if we already went through `updateShapeAndReplaceAllUsesWith` ?
> 
> There are some other uses that replaceAllUses which should not automatically be updated in the ShapeMap, e.g. https://github.com/llvm/llvm-project/blob/main/llvm/lib/Transforms/Scalar/LowerMatrixIntrinsics.cpp#L1487).

Perhaps add a comment to those explaining why they can't use these APIs.  That way the patch is also more clear ;)

https://github.com/llvm/llvm-project/pull/118282