[0/8] Improve vector alias checks for WAR and WAW dependencies

[Richard Sandiford]

For:

  void
  f1 (int *x, int *y)
  {
    for (int i = 0; i < 32; ++i)
      x[i] += y[i];
  }

we check at runtime whether one vector at x would overlap one vector at y.
But in cases like this, the vector code would handle x <= y just fine,
since any write to address A still happens after any read from address A.
The only problem is if x is ahead of y by less than a vector.

The same is true for two writes:

  void
  f2 (int *x, int *y)
  {
    for (int i = 0; i < 32; ++i)
      {
        x[i] = i;
        y[i] = 2;
      }
  }

If y <= x then a vector write at y after a vector write at x would
have the same net effect as the original scalar writes.

Read-after-write cases like:

  int
  f3 (int *x, int *y)
  {
    int res = 0;
    for (int i = 0; i < 32; ++i)
      {
        x[i] = i;
        res += y[i];
      }
    return res;
  }

can cope with x == y, but otherwise don't allow overlap in either
direction.  Since checking for x == y at runtime would require extra
code, we're probably better off sticking with the current overlap test.

An overlap test is also needed if the scalar or vector accesses covered
by the alias check are mixed together, rather than all statements for
the second access following all statements for the first access.

This patch series tracks whether accesses in an alias check are
well-ordered, and also tracks which combination of RAW, WAR and WAW
dependencies the alias check covers.  It then uses a more relaxed
condition for well-ordered WAR and WAW alias checks.

The most important case this allows is equal addresses/indices for WAR
dependencies.  E.g. more realistic instances of functions like f1 can
support both "constructive" and "destructive" operations, where the
destination pointer is explicitly allowed to be the same as a source
pointer or point to an independent object.

The series probably doesn't help other cases that much in practice.
However, the checks involved are just as simple as (and sometimes
slightly simpler than) the corresponding overlap tests, so there should
be no downside to using the more relaxed tests whenever we can.

The main reason for doing this now is that SVE2 has instructions that
detect RAW and WAR/WAW hazards between two vector pointers.  A follow-on
patch adds support for them.

Each patch tested individually on aarch64-linux-gnu and the series
as a whole on x86_64-linux-gnu.  OK to install?

Richard