[PowerPC] Fix shift amount of xxsldwi when performing vector int_to_double

POC
```
// main.c
#include <stdio.h>
#include <altivec.h>
extern vector double foo(vector int s);
int main() {
  vector int s = {0, 1, 0, 4};
  vector double vd;
  vd = foo(s);
  printf("%lf %lf\n", vd[0], vd[1]);
  return 0;
}
// poc.c
vector double foo(vector int s) {
  int x1 = s[1];
  int x3 = s[3];
  double d1 = x1;
  double d3 = x3;
  vector double x = { d1, d3 };
  return x;
}
```
Compiled with `poc.c main.c -mcpu=pwr8 -O3` on BE machine.
Current clang gives
```
4.000000 1.000000
```
while xlc gives
```
1.000000 4.000000
```
Xlc's output should be correct.

Reviewed By: shchenz, #powerpc

Differential Revision: https://reviews.llvm.org/D107428