[GlobalOpt] recognize dead struct fields and propagate values

Summary:
Allow struct fields SRA and dead stores. This works by considering fields accesses from getElementPtr to be considered as a possible pointer root that can be cleaned up.
We check that the variable can be SRA by recursively checking the sub expressions with the new isSafeSubSROAGEP function.

basically this allows the array in following C code  to be optimized out 

struct Expr {
  int a[2];
  int b;
};

static struct Expr e;

int foo (int i)
{
  e.b = 2;
  e.a[i] = 1;
  return e.b;
}


Reviewers: greened, bkramer, nicholas, jmolloy

Reviewed By: jmolloy

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D61911

llvm-svn: 361460