DAGCombiner: Merge store/loads when we have extload/truncstores

This is helps on architectures where i8,i16 are not legal but we have byte, and
short loads/stores. Allowing us to merge copies like the one below on ARM.

copy(char *a, char *b, int n) {
 do {
   int t0 = a[0];
   int t1 = a[1];
   b[0] = t0;
   b[1] = t1;

radar://13536387