radeonsi: work around an MSAA fast stencil clear problem
radeonsi: work around an MSAA fast stencil clear problem

A piglit test (arb_texture_multisample-stencil-clear) has been sent.
This problem was discovered analyzing

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=93767
Reviewed-by: Marek Olšák <8c7344a1abdb103e79ecfd488098373070c3c70e@amd.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler