RFC: Simplify rules for ctz/clz patterns and RTL

[David Edelsohn]

>>>>> Segher Boessenkool writes:

>> I think the cost would be something like:
>> +    case POPCOUNT:
>> +      *total = COSTS_N_INSNS (3);
>> +      return false;

Segher> Is that the cost when using popcountb?  It is a lot more
Segher> expensive when that instruction isn't available (like on
Segher> most current machines).

	Yes, but do we even create POPCOUNT rtx if the insn isn't
supported?  Wouldn't we expand or create libcall early?

David