[mlir][linalg] Change linalg.broadcast `dimensions` attribute to represent added dimensions.

Original [RFC](discourse.llvm.org/t/rfc-primitive-ops-add-broadcastop-to-linalg/66313) defined `dimensions` as a map from input to init, but a discussion in reviews.llvm.org/D138291 concluded that it's more natural for `dimensions` to represent added dims. Also this way is more consistent with `linalg.reduce`.

Differential Revision: https://reviews.llvm.org/D138408