Rewrite the section on trap values to contain a generic description

Rewrite the section on trap values to contain a generic description
of dependence and define trap values in terms of dependence, instead
of trying to cover the concept with a flurry of ad-hoc rules.

The dependence model isn't complete yet, but it's already much more
rigorous than the description it replaces.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@102479 91177308-0d34-0410-b5e6-96231b3b80d8