AMDGPU/InsertWaitcnts: Use foreach loops for inst and wait event types

Summary:
It hides the type casting ugliness, and I happened to have to add a new
such loop (in a later patch).

Reviewers: msearles, rampitec, scott.linder, kanarayan

Subscribers: arsenm, kzhuravl, jvesely, wdng, yaxunl, dstuttard, tpr, t-tye, llvm-commits, hakzsam

Differential Revision: https://reviews.llvm.org/D54227

llvm-svn: 347849