A DAGCombine optimization for mergeing consecutive stores to memory. The optimization

A DAGCombine optimization for mergeing consecutive stores to memory. The optimization
is not profitable in many cases because modern processors perform multiple stores
in parallel and merging stores prior to merging requires extra work. We handle two main cases:

1. Store of multiple consecutive constants:
  q->a = 3;
  q->4 = 5;
In this case we store a single legal wide integer.

2. Store of multiple consecutive loads:
  int a = p->a;
  int b = p->b;
  q->a = a;
  q->b = b;
In this case we load/store either ilegal vector registers or legal wide integer registers.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@165125 91177308-0d34-0410-b5e6-96231b3b80d8