ppc64le: possible SIMD issue casting double to int
Description of problem:
Working with numpy in a ppc64le VM, I ran into a strange double -to casting issue, specifically when casting an array of 1.0 values to 1 values. The numpy folks guided me to a small reproducible test case.

The attached [convert.c](/uploads/2dd7936f4defccf816ffee7c7c002e77/convert.c) creates double and int arrays of length `1 <= n <= 16`. The double array is filled with the value 1.0, and both arrays are passed to a function that converts the value.

With `-O2`, output is as expected (truncated here): 

```
i =  1:   1
i =  2:   1 1
i =  3:   1 1 1
i =  4:   1 1 1 1
i =  5:   1 1 1 1 1
i =  6:   1 1 1 1 1 1
```

With `-O3`, all values that fit into blocks of four become zero:
```
i =  1:   1
i =  2:   1 1
i =  3:   1 1 1
i =  4:   0 0 0 0
i =  5:   0 0 0 0 1
i =  6:   0 0 0 0 1 1
```

I tested this with executables compiled on a physical ppc64le host, where the issue is not reproducible.
Steps to reproduce:
1. `gcc -O2 -o convert convert.c && ./convert`
2. `gcc -O3 -o convert convert.c && ./convert`