[LoopVectorize][SVE] Add tests for vectorising conditional loads of invariant addresses

For loops of the form:

 void foo(int *a, int *cond, short *inv, long long n) {
   for (long long i=0; i<n; ++i) {
     if (cond[i])
       a[i] = *inv;
   }
 }

we can vectorise for SVE using masked gather loads where the array
of pointers is simply a vector splat of 'inv' and the mask comes
from the condition 'cond[i] != 0'.

This patch simply adds tests upstream to defend this capability.

Differential Revision: https://reviews.llvm.org/D98043