[AMDGPU][Waitcnt] As of gfx7, VMEM operations do not increment the export...

[AMDGPU][Waitcnt] As of gfx7, VMEM operations do not increment the export counter and the input registers are available in the next instruction; update the waitcnt pass to take this into account.

Differential Revision: https://reviews.llvm.org/D46067

llvm-svn: 330954