Integrate Jeffery Yasskin's suggestions with respect to

Integrate Jeffery Yasskin's suggestions with respect to
traps flowing through memory references, add some text to
better cover phi nodes and externally-visible side effects,
add an example of instructions being control-dependent
on a trap value, and reword some of the existing trap rules.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@102399 91177308-0d34-0410-b5e6-96231b3b80d8