math/bits: panic when y<=hi in Div

math/bits: panic when y<=hi in Div

Explicitly check for divide-by-zero/overflow and panic with the appropriate
runtime error.  The additional checks have basically no effect on performance
since the branch is easily predicted.

name     old time/op  new time/op  delta
Div-4    53.9ns ± 1%  53.0ns ± 1%  -1.59%  (p=0.016 n=4+5)
Div32-4  17.9ns ± 0%  18.4ns ± 0%  +2.56%  (p=0.008 n=5+5)
Div64-4  53.5ns ± 0%  53.3ns ± 0%    ~     (p=0.095 n=5+5)

Updates #28316

Change-Id: I36297ee9946cbbc57fefb44d1730283b049ecf57
Reviewed-on: https://go-review.googlesource.com/c/144377
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>