Simplify TableGen type-compatibility checks.

Patch by Elior Malul!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@171684 91177308-0d34-0410-b5e6-96231b3b80d8
tblgen, docs: Add initial syntax reference.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@171685 91177308-0d34-0410-b5e6-96231b3b80d8