[LoopVectorize] Enable integer Mul and Add as select reduction patterns

This patch vectorizes Phi node loop reductions for select's whos condition
comes from a floating-point comparison, with its operands being integers
for Add, Sub, and Mul reductions.

Example:

int foo(float *x, int n) {
    int sum = 0;
    for (int i=0; i<n; ++i) {
        float elem = x[i];
        if (elem > 0) {
            sum += 2;
        }
    }
    return sum;
}

This would previously fail to vectorize due to the integer reduction.