[mlir][linalg] Fix `FoldInitTensorWithDimOp` if dim(init_tensor) is static.

It looks like it was a typo. Instead of `*maybeConstantIndex`,
`initTensorOp.getStaticSize(*maybeConstantIndex)` should be used to access the
dim size of the tensor. There is a test for that in `canonicalize.mlir`, but it
was working correctly because `ReplaceStaticShapeDims` was canonicalizing DimOp
before `FoldInitTensorWithDimOp`. So, to make the patterns more "orthogonal",
this case is disabled.

Differential Revision: https://reviews.llvm.org/D109247