[linalg] When removing noop linalg.generics, check that inserting a cast is valid

[linalg] When removing noop linalg.generics, check that inserting a cast is valid

linalg.generic can also take scalars instead of tensors, which
tensor.cast doesn't support. We don't have an easy way to cast between
scalars and tensors so just keep the linalg.generic in those cases.

Differential Revision: https://reviews.llvm.org/D122575