FPU: Make an explicit exponent data path

FPU: Make an explicit exponent data path

With this, the large case statement sets values for a set of control
signals, which then control multiplexers and adders that generate
values for v.result_exp and v.shift.  The plan is for the case
statement to turn into a microcode ROM eventually.

The value of v.result_exp is the sum of two values, either of which
can be negated (but not both).  The first value can be chosen from the
result exponent, A exponent, B exponent arithmetically shifted right
one bit, or 0.  The second value can be chosen from new_exp (which is
r.result_exp - r.shift), B exponent, C exponent or a constant.  The
choices for the constant are 0, 56, the maximum exponent (max_exp) or
the exponent bias for trap-enabled overflow conditions (bias_exp).
These choices are controlled by the signals re_sel1, re_neg1, re_sel2
and re_neg2, and the sum is written into v.result_exp if re_set_result
is 1.

For v.shift we also compute the sum of two values, either of which
can be negated (but not both).  The first value can be chosen from
new_exp, B exponent, r.shift, or 0.  The second value can be chosen
from the A exponent or a constant.  The possible constants are 0, 1,
4, 8, 32, 52, 56, 63, 64, or the minimum exponent (min_exp).  These
choices are controlled by the signals rs_sel1, rs_neg1, rs_sel2 and
rs_neg2.  After the adder there is a multiplexer which selects either
the sum or a shift count for normalization (derived from a count
leading zeroes operation on R) to be written into v.shift.  The
count-leading-zeroes result does not go through the adder for timing
reasons.

In order to simplify the logic and help improve timing, settings of
the control signals have been made unconditional in a state in many
places, even if those settings are only required when some condition
is met.

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