Rather than comparing against both min and max, FIPS prefers comparing
with max - min and adding min. It also does not believe in using
3*range. Align with it, though our old algorithm trivially produces the
same probability distribution on values.

Change-Id: I447cc3608b92ba93706489d702b8d6a68047f491
Reviewed-on: https://boringssl-review.googlesource.com/15045
Reviewed-by: Adam Langley <agl@google.com>
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>