Rather than comparing against both endpoints, subtract the minimum and
rely on unsigned wraparound to do both comparisons at once. This seems
to be slightly faster.

In addition, constant_time_lt_8 becomes much simpler if it can assume
that |a| and |b| have the same MSB. But we can arrange that by casting
up to |crypto_word_t| (which is otherwise happening anyway).

Change-Id: I82bd676e487eb7bb079ba7286df724c1c380bbb4
Reviewed-on: https://boringssl-review.googlesource.com/16445
Reviewed-by: Adam Langley <agl@google.com>