[AMDGPU] Fixed wait count reuse

The code reusing existing wait counts is incorrect since it keeps
adding new operands to an old instruction instead of replacing
the immediate. It was also effectively switched off by the condition
that wait count is not an AMDGPU::S_WAITCNT.

Also switched to BuildMI instead of creating instructions directly.

Differential Revision: https://reviews.llvm.org/D42997

llvm-svn: 324547