[AMDGPU] Flush the vmcnt counter in loop preheaders when necessary

waitcnt vmcnt instructions are currently generated in loop bodies before using
values loaded outside of the loop. In some cases, it is better to flush the
vmcnt counter in a loop preheader before entering the loop body. This patch
detects these cases and generates waitcnt instructions to flush the counter.

Reviewed By: foad

Differential Revision: https://reviews.llvm.org/D115747