[SDAG] Remove a giant pile of asserts that may have helped track down
a bug in 2010 when they were added but are adding no value today.

In fact, they are utter lies. NodeAllocator is used to allocate almost
all of these node types. I don't know what we were trying to assert
here, and the docs don't give any answer. Until we once again stumble
upon a bug needing help, let's clear the path for improvements.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213610 91177308-0d34-0410-b5e6-96231b3b80d8
[SDAG] Refactor the code for inserting a newly allocated SDNode into the
DAG into a helper function.

This adds a trip through the (very minimal) verification logic in
a bunch of places that were missing it, but shouldn't have any other
impact outside of refactoring. I'm hoping to use this to do more clever
things when DAG nodes are inserted into the graph.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213612 91177308-0d34-0410-b5e6-96231b3b80d8