[AMDGPU] Avoid flushing the vmcnt counter in loop preheaders if not necessary

One of the conditions to flush the vmcnt counter in loop preheaders is: The loop
contains a use of a vgpr that is defined out of the loop. The code currently
checks if a waitcnt is needed by looking at the score of that vgpr in the score
brackets. This is not enough and may cause the generation of an unnecessary
vmcnt flush. This patch fixes that case.

Differential Revision: https://reviews.llvm.org/D130313