Clean up static analyzer warnings.

Clang's static analyzer found several potential cases of undefined
behavior, use of un-initialized values, and potentially null pointer
dereferences in tablegen, Support, MC, and ADT. This cleans them up
with specific assertions on the assumptions of the code.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224154 91177308-0d34-0410-b5e6-96231b3b80d8