[sa]: Make component references must-defs when possible

[Daniel Berlin <dberlin at dberlin dot org>]

This patch fixes the long standing problem in tree-ssa that we would 
generate different code for the following two loops, and have extra 
needless stores in the second loop:

int x; int y;
struct { int x; int y; } global;
int foo() {
        int i;
        for ( i=0; i<10; i++)
                y += x*x;
        for ( i=0; i<10; i++)
                global.y += global.x*global.x;
}


This was because even with the previous patch to make the component refs 
accessing different fields into vuses/v_may_defs of different variables, 
we still couldn't eliminate the needless stores because they were only 
may-defs, not must-defs.

The attached patch makes component-refs into must-defs when possible, 
resulting in a lot more component accesses being eliminated.

With it, the code produced for the above two loops is identical, having 
the same number of loads and stores from the various variables.

We currently don't make them must-defs when we have overlaps or array 
references in the component reference.

This is conservatively correct. We could be a bit more aggressive than we 
are now.
For the case of overlaps, we could do must-defs for every fake variable 
that is  completely covered by the access. For array refs, we could do it 
if the  array reference was to a constant offset.

Bootstrapped and regtested on i686-pc-linux-gnu.  Committed to structure 
aliasing branch.

--Dan

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