[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` 

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