FPU: Decide on A input selection a cycle earlier

FPU: Decide on A input selection a cycle earlier

This moves opsel_a into the reg_type record, meaning that the A
multiplexer input now needs to be decided a cycle earlier.  This helps
timing by eliminating the combinatorial path from r.state and other
things to opsel_a and thence to in_a and result.

This means that some things now take an extra cycle, in particular
some of the exception cases such as when one or both operands are
NaNs.  The NaN handling has been moved out to its own state, which
simplifies the logic for exception cases in other places.  Additions
or subtractions where FRB's exponent is smaller than FRA's will
also take an extra cycle.

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