AMDGPU/InsertWaitcnt: Consistently use uint32_t for scores / time points

Summary:
There is one obsolete reference to using -1 as an indication of "unknown",
but this isn't actually used anywhere.

Using unsigned makes robust wrapping checks easier.

Reviewers: msearles, rampitec, scott.linder, kanarayan

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

Differential Revision: https://reviews.llvm.org/D54230

llvm-svn: 347852