Rearrange the comments, control flow, and variable names; no

Rearrange the comments, control flow, and variable names; no
functionality changed.

Evan's commit r168970 moved the code that the primary comment in this
function referred to to the other end of the function without moving the
comment, and there has been a steady creep of "boolean" logic in it that
is simpler if handled via early exit. That way each special case can
have its own comments. I've also made the variable name a bit more
explanatory than "AllFit". This is in preparation to fix the
non-deterministic output of this function.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@168988 91177308-0d34-0410-b5e6-96231b3b80d8