Document the fact that the verifier currently requires the

Document the fact that the verifier currently requires the
top bit of a ValueType to be zero.  Enforce this by ensuring
an assertion failure if someone tries to create a ValueType
without this property.  I chose this minimal approach rather
than a more official integration of the notion of reserved
bits into ValueType because I'm hoping that the verifier will
be changed to no longer require this :)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@43031 91177308-0d34-0410-b5e6-96231b3b80d8