execute1: Improve timing on comparisons

execute1: Improve timing on comparisons

Using the main adder for comparisons has the disadvantage of creating
a long path from the CA/OV bit forwarding to v.busy via the carry
input of the adder, the comparison result, and determining whether a
trap instruction would trap.  Instead we now have dedicated
comparators for the high and low words of a_in vs. b_in, and combine
their results to get the signed and unsigned comparison results.

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