[PowerPC] Provide doubleword vector predicate form comparisons on Power7

There are two reasons this shouldn't be restricted to Power8 and up:
1. For XL compatibility
2. Because clang will expand comparison operators to these intrinsics*

*Without this patch, the following causes a selection error:

int test(vector signed long a, vector signed long b) {
  return a < b;
}

This patch provides the handling for the intrinsics in the back
end and removes the Power8 guards from the predicate functions
(vec_{all|any}_{eq|ne|gt|ge|lt|le}).