Thank you for the report!

Can you try with Mesa from git master?

The issue is that in Mesa 17.2.2 (which you have according to glxinfo),
si_state_draw.c:1305 corresponds to

   1304                         index_offset -= start_offset;
   1305                 } else if (sctx->b.chip_class <= CIK &&
   1306                            r600_resource(indexbuf)->TC_L2_dirty) {

and it's basically impossible to get a crash there, so I suspect the Gentoo
build actually applies some other patches.

Something random you could try is starting KWin with GALLIUM_THREAD=0.