After JDK-8275317, C2's SLP vectorizer has supported type conversion
between the same data size. We can also support conversions between
different data sizes like:
int <-> double
float <-> long
int <-> long
float <-> double

A typical test case:

int[] a;
double[] b;
for (int i = start; i < limit; i++) {
    b[i] = (double) a[i];
}