[PowerPC] Fix invalid cast for vector shuffles when lowering to the xxsplti32dx instruction.

When lowering vector shuffles into the xxsplti32dx instruction on Power10, we
canonicalize the right operand to be a BUILD_VECTOR and as a result, get the
commuted vector shuffle node.

However, a vector shuffle will not always be returned as the result for a
commuted vector shuffle. In such a scenario, this patch updates the original
cast of a shuffle into a dyn_cast<> and checks if the shuffle is a valid vector
shuffle node prior to obtaining the commuted shuffle mask.

This patch also adds a new test case that demonstrates this scenario (primarily
seen on 32-bit), and was originally a crash prior to this fix.

Differential Revision: https://reviews.llvm.org/D135024