[LoopVectorize] Fix bug where predicated loads/stores were dropped

This commit fixes a bug where the loop vectoriser fails to predicate
loads/stores when interleaving for targets that support masked
loads and stores.

Code such as:

     1  void foo(int *restrict data1, int *restrict data2)
     2  {
     3    int counter = 1024;
     4    while (counter--)
     5      if (data1[counter] > data2[counter])
     6        data1[counter] = data2[counter];
     7  }

... could previously be transformed in such a way that the predicated
store implied by:

    if (data1[counter] > data2[counter])
       data1[counter] = data2[counter];

... was lost, resulting in miscompiles.

This bug was causing some tests in llvm-test-suite to fail when built
for SVE.

Differential Revision: https://reviews.llvm.org/D99569