[TableGen] Fix reporting from CodeGenSchedModels::checkCompleteness().

Make the check for a complete SchedModel work as expected: report any
supported instruction not having scheduler info.

For unclear reasons there was a variable 'HadCompleteModel' that caused
e.g. new instructions for a new subtarget not to be reported. This variable
is now simply removed as all in-tree targets seem to build fine without it.

Review: Simon Pilgrim

Differential Revision: https://reviews.llvm.org/D118628