decode: Work out ispr1/ispr2 in parallel with decode ROM lookup

decode: Work out ispr1/ispr2 in parallel with decode ROM lookup

This makes the logic that calculates which SPRs are being accessed
work in parallel with the instruction decode ROM lookup instead of
being dependent on the opcode found in the decode ROM.  The reason
for doing that is that the path from icache through the decode ROM
to the ispr1/ispr2 fields has become a critical path.

Thus we are now using only a very partial decode of the instruction
word in the logic for isp1/isp2, and we therefore can no longer rely
on them being zero in all cases where no SPR is being accessed.
Instead, decode2 now ignores ispr1/ispr2 in all cases except when the
relevant decode.input_reg_a/b or decode.output_reg_a is set to SPR.

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