Sharing hard registers

[law@redhat.com]

It may be necessary to back out the last patch which makes gen_rtx_REG
share hard registers in some circumstances.

I've run into a problem in at least the rs6000 port which assumes hard
registers are never shared.  I'm planning to fix the rs6000, but we need 
to keep an eye out for problems -- it won't take more than one or two to
make me want to revert that part of the patch.

jeff