TableGen: Add strict assertions to sanity check earlier type checking

Summary:
Both of these errors should have been caught by type-checking during
parsing.

Change-Id: I891087936fd1a91d21bcda57c256e3edbe12b94d

Reviewers: arsenm, craig.topper, tra, MartinO

Subscribers: wdng, llvm-commits

Differential Revision: https://reviews.llvm.org/D43558

llvm-svn: 325800