[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.

llvm-svn: 213610