[AMDGPU] Don't flush vmcnt for loops with use/def pairs

Conditions for hoisting vmcnt with flat instructions should be similar to VMEM.
If there are use/def pairs in a loop body we cannot guarantee that hosting the
waitcnt will be profitable. Better heuristics are needed to analyse whether
gains from avoiding waitcnt in loop bodys outweighs waiting for loads in the
preheader.

Reviewed By: foad

Differential Revision: https://reviews.llvm.org/D151126