[AMDGPU] GFX11: automatically release VGPRs at the end of the shader

GFX11 has a new message type MSG_DEALLOC_VGPRS which can be used to
release a shader's VGPRs. Sending this at the end of a shader (just
before the s_endpgm) can help overall system performance in cases where
the s_endpgm would have to wait for outstanding VMEM stores to complete
before releasing the VGPRs.

Differential Revision: https://reviews.llvm.org/D128442