[mlir][Vector] Pattern to linearize broadcast (#163845)
The PR https://github.com/llvm/llvm-project/pull/162167 removed a
pattern to linearize vector.splat, without adding the equivalent pattern
for vector.broadcast. This PR adds such a pattern, hopefully brining
vector.broadcast up to full parity with vector.splat that has now been
removed.
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Signed-off-by: James Newling <james.newling@gmail.com>
