[LangRef] Clarify poison semantics

I find the current documentation of poison somewhat confusing,
mainly because its use of "undefined behavior" doesn't seem to
align with our usual interpretation (of immediate UB). Especially
the sentence "any instruction that has a dependence on a poison
value has undefined behavior" is very confusing.

Clarify poison semantics by:

 * Replacing the introductory paragraph with the standard rationale
   for having poison values.
 * Spelling out that instructions depending on poison return poison.
 * Spelling out how we go from a poison value to immediate undefined
   behavior and give the two examples we currently use in ValueTracking.
 * Spelling out that side effects depending on poison are UB.

Differential Revision: https://reviews.llvm.org/D63044

llvm-svn: 363320