This PR ensures parity in folding/canonicalizing of vector.broadcast
(from a scalar) and vector.splat. This means that by using
vector.broadcast instead of vector.splat (which is currently
deprecated), there is no loss in optimizations performed. All tests
which were previously checking folding/canonicalizing of vector.splat
are now done for vector.broadcast. The vector.splat canonicalization
tests are now in a separate file, ready for removal when, in the future,
we remove vector.splat completely.
This PR also adds a canonicalizer to vector.splat to always convert it
to vector.broadcast. This is to reduce the 'traffic' through
vector.splat.
There is a chance that this PR will break downstream users who create/expect
for vector.splat. Changing all such logic to work just vector.broadcast instead
should fix.
a96d8aed98