[UpdateTestChecks] Emit warning when invalid value for -check-prefix(es) option

Summary:
The script is silent for the following issue:
FileCheck %s -check-prefix=CHECK,POPCOUNT

FileCheck will catch it later, but I think we can warn here too.

Now it warns:
 ./update_llc_test_checks.py file.ll 
WARNING: Supplied prefix 'CHECK,POPCOUNT' is invalid. Prefix must contain only alphanumeric characters, hyphens and underscores. Did you mean --check-prefixes=CHECK,POPCOUNT?



Reviewers: lebedev.ri, spatel, RKSimon, craig.topper, nikic, gbedwell

Reviewed By: RKSimon

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64589

llvm-svn: 367244