[llvm-mca] Simplify eventing by adding an onEvent templated method.

[llvm-mca] Simplify eventing by adding an onEvent templated method.

Summary:
This patch eliminates some redundancy in iterating across Listeners for the
Instruction and Stall HWEvents, by introducing a template onEvent routine.
This change was suggested by @courbet in https://reviews.llvm.org/D48576.  I
 hope that this patch addresses that suggestion appropriately.  I do like this
change better than what we had previously.

Reviewers: andreadb, courbet, RKSimon

Reviewed By: andreadb, courbet

Subscribers: javed.absar, tschuett, gbedwell, llvm-commits, courbet

Differential Revision: https://reviews.llvm.org/D48672

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@336916 91177308-0d34-0410-b5e6-96231b3b80d8