Move insn_codes for mcrfs, mtfsb0/1 and mtfsfi

Move insn_codes for mcrfs, mtfsb0/1 and mtfsfi

This moves the insn_code values for mcrfs, mtfsb0/1 and mtfsfi into
the region used for floating-point instructions.  This means that in
no-FPU implementations, they will get turned into illegal instructions
in predecode.  We then don't need the code in execute1 that makes FP
instructions illegal in no-FPU implementations.

We also remove the NONE value for unit_t, since it was only ever used
with insn_type = OP_ILLEGAL, and the check for unit = NONE was
redundant with the check for insn_type = OP_ILLEGAL.  Thus the check
for unit = NONE is no longer needed and is removed here.

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