[AMDGPU] Remove unnecessary s_waitcnt between VMEM loads

[AMDGPU] Remove unnecessary s_waitcnt between VMEM loads

VMEM loads of the same type (sampler vs no sampler) are guaranteed to
write their result registers in order, so there is no need for an
s_waitcnt even if they write to overlapping vgprs.

Differential Revision: https://reviews.llvm.org/D79176