[TableGen] Fix negation of simple predicates

Simple predicates, such as those defined by `CheckRegOperandSimple` or
`CheckImmOperandSimple`, were not being negated when used with `CheckNot`.

This change fixes this issue by defining the previously declared methods to
handle simple predicates.

Differential revision: https://reviews.llvm.org/D55089

llvm-svn: 348034