radv: enable conditional discard optimisation on radv.
radv: enable conditional discard optimisation on radv.

This fixes a bunch of GPU hangs introduced in some CTS
tests like
dEQP-VK.memory.pipeline_barrier.host_write_uniform_buffer.65536

It works around an issue seen in the LLVM backend, but
also makes the radv code work more like the radeonsi stack.

Cc: "13.0" <mesa-stable@lists.freedesktop.org>

Signed-off-by: Dave Airlie <f2295d84e358395675bc8031be58672073ae065e@redhat.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler