Add a hack (mirroring llvm-gcc) to pointer difference

codegen to compile:

int test(int *A, int *B) {
  return A-B;
}

into:

_test:
        movl 4(%esp), %eax
        subl 8(%esp), %eax
        sarl $2, %eax
        ret

instead of:

_test:
        movl 4(%esp), %eax
        subl 8(%esp), %eax
        movl %eax, %ecx
        sarl $31, %ecx
        shrl $30, %ecx
        addl %ecx, %eax
        sarl $2, %eax
        ret

llvm-svn: 39902