[AMDGPU/MemOpsCluster] Implement new heuristic for computing max mem ops cluster size

Summary:
Make use of both the - (1) clustered bytes and (2) cluster length, to decide on
the max number of mem ops that can be clustered. On an average, when loads
are dword or smaller, consider `5` as max threshold, otherwise `4`. This
heuristic is purely based on different experimentation conducted, and there is
no analytical logic here.

Reviewers: foad, rampitec, arsenm, vpykhtin

Reviewed By: rampitec

Subscribers: llvm-commits, kerbowa, hiraditya, t-tye, Anastasia, tpr, dstuttard, yaxunl, nhaehnle, wdng, jvesely, kzhuravl, thakis

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D82393