RFC: Simplify rules for ctz/clz patterns and RTL

[Zack Weinberg]

Richard Kenner wrote:
>> * Since no one uses it, we rip out all support for the ffs pattern and
>> expression.
> 
> There's an ffs builtin!  How do we know who uses it?

I am not proposing to remove the built-in (i.e. the language visible 
__builtin_ffs() function); only the RTL expression (ffs:MODE ...) and 
the named machine description pattern "ffs<mode>".

> Moreover, expmed uses it as an option in expanding some comparisons.

I see no code in expmed.c that uses either the RTL expression or the 
named pattern.  It does use the optab, but (to reiterate) use of the 
ffs_optab never generates RTL that uses (ffs:MODE).

zw