Pre-decode instructions when writing them to icache

Pre-decode instructions when writing them to icache

This splits out the decoding done in the decode0 step into a separate
predecoder, used when writing instructions into the icache.  The
icache now holds 36 bits per instruction rather than 32.  For valid
instructions, those 36 bits comprise the bottom 26 bits of the
instruction word, a 9-bit insn_code value (which uniquely identifies
the instruction), and a zero in the MSB.  For illegal instructions,
the MSB is one and the full instruction word is in the bottom 32 bits.
Having the full instruction word available for illegal instructions
means that it can be printed in the log when simulating, or in future
could be placed in the HEIR register.

If we don't have an FPU, then the floating-point instructions are
regarded as illegal.  In that case, the insn_code values would fit
into 8 bits, which could be used in future to reduce the size of
decode_rom from 512 to 256 entries.

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