[SchedModel] Remove instregex entries that don't match any instructions

This patch throws a fatal error if an instregex entry doesn't actually match any instructions. This is part of the work to reduce the compile time impact of increased instregex usage (PR35955), although the x86 models seem to be relatively clean.

All the cases I encountered have now been fixed in trunk and this will ensure they don't get reintroduced.

Differential Revision: https://reviews.llvm.org/D44687

llvm-svn: 328459