[RFC] Kill gen_sequence

[Jan Hubicka]

> refer to the same hard register -- but is it a requirement that each rtx
> for a hard register be distinct?  If we can share hard register rtxs of
> the same mode, then we can eliminate a ton of calls to gen_rtx_REG and
> ggc_alloc.

We already do create shared instances of hard registers.  Most notable
ones are frame_pointer_rtx and friends.  These also needs to remain
unique, so we can't use simple hash table and unionize all hard
registers with same code, but I don't think it is problem to avoid
gen_rtx_REG and create shared instances.

Honza
> 
> jeff