[CodeGen] assume max/default throughput for unspecified instructions

This is a fix for the problem arising in D47374 (PR37678):
https://bugs.llvm.org/show_bug.cgi?id=37678

We may not have throughput info because it's not specified in the model 
or it's not available with variant scheduling, so assume that those
instructions can execute/complete at max-issue-width.

Differential Revision: https://reviews.llvm.org/D47723

llvm-svn: 334055