isl: Take the slice0_extent shortcut for interleaved MSAA
isl: Take the slice0_extent shortcut for interleaved MSAA

The shortcut works just fine for MSAA and the comment even says so.

Reviewed-by: Nanley Chery <d78cd5d33e98a5581566f188959f731bd0b1c0fd@intel.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler