[RISCV] Remove now redundant isElementRotate shuffle lowering [NFCI] (#128064)

This is the first cleanup following the introduction of the new
isMaskedSlidePair lowering in 43f2968. As mentioned in that review, the
new code is a generalization of the existing isElementRotate, but was
staged to make diffs manageable.

This change removes isElementRotate. The tricky bit is making sure that
a) the same shuffles hit the same lowering paths (in particular, two
element shuffles can match multiple ways) and 2) avoiding DAG
canonicalization changes.

0 files changed, 0 insertions, 0 deletions