RFA: fix avoid random results from do_add

[Joern Rennecke]

> 
> > tree.c:iterative_hash_expr line 3848:
> > 
> >       else if (code == REAL_CST)
> >         val = iterative_hash (TREE_REAL_CST_PTR (t),
> >                               sizeof (REAL_VALUE_TYPE), val);
> 
> P.S.: An alternative approach would be to call real_hash there.
> However, I couldn't empirically test if it fixes the original problem
> now - since the problem depends on memory allocation / freeing
> patterns, the 'test case' (a library file from the gcc build) was
> dependent on pathname lengths and the exact invocation options
> (e.g. --save-temps would make the compilation succeed).

P.P.S.: apparently I have a fresh test case after all.  Last night's sh64-elf
build seems to have hit the problem again:

/swbuild/nightly/2004-06-06/sh64-elf/gcc/xgcc -B/swbuild/nightly/2004-06-06/sh64
-elf/gcc/ -nostdinc -B/swbuild/nightly/2004-06-06/sh64-elf/sh64-elf/nofpu/media6
4/newlib/ -isystem /swbuild/nightly/2004-06-06/sh64-elf/sh64-elf/nofpu/media64/n
ewlib/targ-include -isystem /swbuild/nightly/2004-06-06/srcw/newlib/libc/include
 -B/usr/local/sh64-elf/bin/ -B/usr/local/sh64-elf/lib/ -isystem /usr/local/sh64-
elf/include -isystem /usr/local/sh64-elf/sys-include -L/swbuild/nightly/2004-06-
06/sh64-elf/ld  -m5-64media-nofpu -DPACKAGE=\"newlib\" -DVERSION=\"1.12.0\"  -I.
 -I../../../../../../../srcw/newlib/libm/math  -I../../../../../../../srcw/newli
b/libm/math/../common -O2 -DHAVE_GETTIMEOFDAY -fno-builtin    -O2 -g -O2  -O2 -g
 -O2  -m5-64media-nofpu -c ../../../../../../../srcw/newlib/libm/math/ef_rem_pio
2.c
../../../../../../../srcw/newlib/libm/math/ef_rem_pio2.c: In function `__ieee754
_rem_pio2f':
../../../../../../../srcw/newlib/libm/math/ef_rem_pio2.c:104: internal compiler 
error: in avail_expr_eq, at tree-ssa-dom.c:3158