RFC: Adding more expensive tree expression list in stmt_cost

[Andrew Pinski <pinskia at physics dot uc dot edu>]

FLOAT_EXPR and FIX_TRUNC_EXPR are both expensive so they really
should be added to this list.

Take the following code:
int f(float a, int *b)
{
  int i;
  for(i = 0; i<1000;i++)
    *b = a;
}

Compile with -O1 and we get on PPC:
_f:
        fctiwz f0,f1
        addi r2,r1,-4
        stfiwx f0,0,r2
        lwz r2,-4(r1)
        li r0,1000
        mtctr r0
L2:
        stw r2,-8(r1)
        bdnz L2
        stw r2,0(r4)
        blr

Notice the loop and extra store (to the stack), if we change LIM to pull
out the cast (FIX_TRUNC_EXPR), we get:
_f:
        fctiwz f0,f1
        stfiwx f0,0,r4
        blr

Which is much better and there is no loop.  Also this is already done  
by PRE
at -O2 but it would be nice if LIM did it also.  I should mention  
that PRE
does not care about cost really and always pulls out the calculation  
so maybe
LIM should be doing that instead.

Thanks,
Andrew Pinski