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.
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