[SVE][LoopVectorize] Verify support for vectorizing loops with invariant loads

D95598 added a cost model for broadcast shuffle, which should enable loops
such as the following to vectorize, where the load of b[42] is invariant
and can be done using a scalar load + splat:

  for (int i=0; i<n; ++i)
    a[i] = b[i] + b[42];

This patch adds tests to verify that we can vectorize such loops.

Reviewed By: joechrisellis

Differential Revision: https://reviews.llvm.org/D98506