xilinx: Improve flip-flop handling.

This adds support for infering more kinds of flip-flops:

- FFs with async set/reset and clock enable
- FFs with sync set/reset
- FFs with sync set/reset and clock enable

Some passes have been moved (and some added) in order for dff2dffs to
work correctly.

This gives us complete coverage of Virtex 6+ and Spartan 6 flip-flop
capabilities (though not latch capabilities).  Older FPGAs also support
having both a set and a reset input, which will be handled at a later
data.

tests/arch/xilinx/macc.ys

@@ -23,9 +23,10 @@ miter -equiv -flatten -make_assert -make_outputs gold gate miter
 sat -verify -prove-asserts -seq 10 -show-inputs -show-outputs miter
 design -load postopt # load the post-opt design (otherwise equiv_opt loads the pre-opt design)
 cd macc2 # Constrain all select calls below inside the top module
+
 select -assert-count 1 t:BUFG
 select -assert-count 1 t:DSP48E1
 select -assert-count 1 t:FDRE
 select -assert-count 1 t:LUT2
-select -assert-count 41 t:LUT3
+select -assert-count 40 t:LUT3
 select -assert-none t:BUFG t:DSP48E1 t:FDRE t:LUT2 t:LUT3 %% t:* %D