So this ends up looking a lot like the bits that I had to revert several 
weeks ago :-)

The core issue we have is given an INSN the generic code will cost the 
SET_SRC and SET_DEST and sum them.  But that's far from ideal on a RISC 
target.

For a register destination, the cost can be determined be looking at 
just the SET_SRC.  Which is precisely what this patch does.  When the 
outer code is an INSN and we're presented with a SET we take one of two 
paths.

If the destination is a register, then we recurse just on the SET_SRC 
and we're done.  Otherwise we fall back to the existing code which sums 
the cost of the SET_SRC and SET_DEST.  That fallback path isn't great 
and probably could be further improved (just costing SET_DEST in that 
case is probably quite reasonable).

The difference between this version and the bits that slipped through by 
accident several weeks ago is that old version mis-used the API due to a 
thinko on my part.

This tightens up various zicond tests to avoid undesirable matching.

This has been tested on rv64gc -- the only difference it makes on the 
testsuite is the new tests (included in this patch) flip from failing to 
passing.

Pushed to the trunk.

Jeff