[AMDGPU] Do not release VGPRs at -O0

This was an oversight when the GFX11 early release VGPRs optimization
was reimplemented in D153279.

Sending the DEALLOC_VGPRS message is a performance optimization so there
is no need to do it at -O0. In addition it makes some kinds of post
mortem debugging hard or impossible, since VGPR values are no longer
available to inspect at the s_endpgm instruction.

Differential Revision: https://reviews.llvm.org/D157599

(cherry picked from commit 3091bdb8)