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.