predecode: Work around apparent yosys/nextpnr bug

predecode: Work around apparent yosys/nextpnr bug

This rearranges the synchronous process here to avoid setting fields
of pred(i) to zero or INSN_illegal when valid_in is '0'.
Experimentally, on ECP5 this acts like an asynchronous reset rather
than a synchronous reset.

Instead, handle possible indeterminate input for simulation by making
the maj_predecode and row_predecode fields of predec_t be unsigned
rather than insn_code (an enumerated type), and setting them to X when
the input word is indeterminate.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>