Second ping for cannot_force_const_mem & LEGITIMATE_CONSTANT_P changes

[Richard Sandiford]

Ping for these two changes:

    http://gcc.gnu.org/ml/gcc-patches/2011-04/msg00194.html
    http://gcc.gnu.org/ml/gcc-patches/2011-04/msg00195.html

They just add a mode argument to TARGET_CANNOT_FORCE_CONST_MEM
and LEGITIMATE_CONSTANT_P (turning the latter into a hook).
This is directly needed for:

    http://gcc.gnu.org/ml/gcc-patches/2011-04/msg00200.html

but it seems like useful information for the back-end to have
in any case.

Richard