Symbolically evaluate constant expressions like &A[123] - &A[4].f.

This occurs in C++ code like:

#include <iostream>
#include <iterator>
int a[] = { 1, 2, 3, 4, 5 };
int main() {
  using namespace std;
  copy(a, a + sizeof(a)/sizeof(a[0]), ostream_iterator<int>(cout, "\n"));
  return 0;
}

Before we would decide the loop trip count is:
sdiv (i32 sub (i32 ptrtoint (i32* getelementptr ([5 x i32]* @a, i32 0, i32 5) to i32), i32 ptrtoint ([5 x i32]* @a to i32)), i32 4)

Now we decide it is "5".  Amazing.

This code will need to be refactored, but I'm doing that as a separate
commit.

llvm-svn: 33665