var-tracking.c: Fix possible use of uninitialized variable pre

While bootstrapping GCC on S/390 the following warning/error is raised:

gcc/var-tracking.c:10239:34: error: 'pre' may be used uninitialized in this function [-Werror=maybe-uninitialized]
10239 |       VTI (bb)->out.stack_adjust += pre;
      |                                  ^

The lines of interest are:

  HOST_WIDE_INT pre, post = 0;
  // ...
  if (!frame_pointer_needed)
    {
      insn_stack_adjust_offset_pre_post (insn, &pre, &post);
      // ...
    }

  // ...
  adjust_insn (bb, insn);

  if (!frame_pointer_needed && pre)
    VTI (bb)->out.stack_adjust += pre;

Both if statements depend on global variable frame_pointer_needed.  In function
insn_stack_adjust_offset_pre_post local variable pre is initialized.  The
problematic part is the function call between both if statements.  Since
adjust_insn also calls functions which are defined in a different compilation
unit, we are not able to prove that global variable frame_pointer_needed is not
altered by adjust_insn and its siblings.  Thus we must assume that
frame_pointer_needed may be true before the call and false afterwards which
renders the warning true (admitted the location hint is not totally perfect).
By initialising pre we silence the warning.

gcc/ChangeLog:

2020-04-30  Stefan Schulze Frielinghaus  <stefansf@linux.ibm.com>

        * var-tracking.c (vt_initialize): Move variables pre and post
        into inner block and initialize both in order to fix warning
        about uninitialized use.  Remove unnecessary checks for
        frame_pointer_needed.