[AMDGPU] Make note of existing waitcnt instrs; this is add-on work related to...

[AMDGPU] Make note of existing waitcnt instrs; this is add-on work related to suppression of redundant waitcnt instrs. It is necessary to make note of these existing waitcnt instrs so that we do not fall into an infinite loop when handling loops. Also, [NFC] some minor code clean-up.

llvm-svn: 325524