Remove CONST_DOUBLE_MEM

[Richard Kenner]

    Have you tested targets that have per-function constant pools, like
    arm, sh, mcore, or mips16?  In that case, 0.0 and 1.0 may give you
    problems, since they are canonical rtl, but would need different
    memory addresses in an inner and outer function.

varasm.c acts like *all* targets have per-function constant pools, so that's
not an issue. Indeed, it's the need for per-constant pools that causes the
problem I'm fixing in the first place.