Mark unlikely parts of division

[chfast]

Mark add-back step and division overflow case as unlikely.

Clang 10:

div<uint256, udivrem>/64_mean                   -0.0029         -0.0029            32            32            32            32
div<uint256, udivrem>/128_mean                  -0.0129         -0.0129            32            32            32            32
div<uint256, udivrem>/192_mean                  -0.0480         -0.0480            36            34            36            34
div<uint256, udivrem>/256_mean                  -0.0595         -0.0595            33            31            33            31
div<uint512, udivrem>/64_mean                   +0.0039         +0.0039            57            57            57            57
div<uint512, udivrem>/128_mean                  +0.0176         +0.0176            56            57            56            57
div<uint512, udivrem>/192_mean                  -0.0513         -0.0513            78            74            78            74
div<uint512, udivrem>/256_mean                  -0.0383         -0.0383            77            74            77            74

GCC10:

div<uint256, udivrem>/64_mean                   -0.0097         -0.0097            25            25            25            25
div<uint256, udivrem>/128_mean                  -0.0045         -0.0045            31            30            31            30
div<uint256, udivrem>/192_mean                  +0.0085         +0.0085            36            36            36            36
div<uint256, udivrem>/256_mean                  +0.0078         +0.0079            32            32            32            32
div<uint512, udivrem>/64_mean                   +0.0000         +0.0000            39            39            39            39
div<uint512, udivrem>/128_mean                  +0.0159         +0.0159            51            52            51            52
div<uint512, udivrem>/192_mean                  -0.0436         -0.0436            78            75            78            75
div<uint512, udivrem>/256_mean                  -0.0574         -0.0574            77            73            77            73

[codecov-commenter]

Codecov Report

Merging #147 into master will increase coverage by 0.12%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #147      +/-   ##
==========================================
+ Coverage   66.60%   66.72%   +0.12%     
==========================================
  Files          10       11       +1     
  Lines         548      541       -7     
  Branches       50       55       +5     
==========================================
- Hits          365      361       -4     
+ Misses        183      180       -3