Looks like some stack stomping is preventing a full backtrace in gdb:

deqp/modules/gles2 $ KNOB_SINGLE_THREADED=1 DISPLAY=:0 GALLIUM_DRIVER=swr LIBGL_ALWAYS_SOFTWARE=1 LD_LIBRARY_PATH=/home/ilia/install/lib gdb --args ./deqp-gles2 --deqp-visibility=hidden --deqp-case=dEQP-GLES2.functional.shaders.invariance.highp.loop_2
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Reading symbols from ./deqp-gles2...(no debugging symbols found)...done.
(gdb) r
Starting program: /home/ilia/src/deqp/modules/gles2/deqp-gles2 --deqp-visibility=hidden --deqp-case=dEQP-GLES2.functional.shaders.invariance.highp.loop_2
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib64/libthread_db.so.1".
dEQP Core git-aa4099c48f58c93d951b64451ef87adb31fce406 (0xaa4099c4) starting..
  target implementation = 'X11 GLX'
SWR detected AVX2

Test case 'dEQP-GLES2.functional.shaders.invariance.highp.loop_2'..
Vertex shader compile time = 4.425000 ms
Fragment shader compile time = 0.368000 ms
Link time = 0.982000 ms
Vertex shader compile time = 0.653000 ms
Fragment shader compile time = 0.354000 ms
Link time = 0.886000 ms
vert shader  0x7ffff7fcf000
frag shader  0x7ffff7fcd000
vert shader  0x7ffff7fcb000
frag shader  0x7ffff7fc9000
fetch shader 0x7ffff7fc7000
vert shader  0x7ffff7fc5000
frag shader  0x7ffff7fc3000
./rasterizer/core/clip.h(429): ASSERT: numEmittedVerts <= 7
        ClipSimd
        Unexpected vertex count from clipper.

Program received signal SIGTRAP, Trace/breakpoint trap.
Clipper<3u>::ClipSimd(float __vector const&, float __vector const&, PA_STATE&, long long __vector const&, long long __vector const&) (this=0x7ffffffcfce0, 
    vPrimMask=..., vClipMask=..., pa=..., vPrimId=..., vViewportIdx=...)
    at ./rasterizer/core/clip.h:431
431                 uint32_t numEmittedPrims = GetNumPrims(clipTopology, numEmittedVerts);
(gdb) bt
#0  Clipper<3u>::ClipSimd(float __vector const&, float __vector const&, PA_STATE&, long long __vector const&, long long __vector const&) (this=0x7ffffffcfce0, 
    vPrimMask=..., vClipMask=..., pa=..., vPrimId=..., vViewportIdx=...)
    at ./rasterizer/core/clip.h:431
Backtrace stopped: Cannot access memory at address 0x7fffbf20cb0f

OTOH with valgrind I get this on exit:

./rasterizer/core/clip.h(429): ASSERT: numEmittedVerts <= 7
        ClipSimd
        Unexpected vertex count from clipper.
==9673== 
==9673== Process terminating with default action of signal 5 (SIGTRAP)
==9673==    at 0x10243787: Clipper<3u>::ClipSimd(float __vector(8) const&, float __vector(8) const&, PA_STATE&, long long __vector(4) const&, long long __vector(4) const&) (clip.h:431)
==9673==    by 0x1024101F: Clipper<3u>::ExecuteStage(PA_STATE&, simdvector*, unsigned int, long long __vector(4), long long __vector(4)) (clip.h:558)
==9673==    by 0x1023F6A0: ClipTriangles(DRAW_CONTEXT*, PA_STATE&, unsigned int, simdvector*, unsigned int, long long __vector(4), long long __vector(4)) (clip.cpp:187)
==9673==    by 0x10288CF7: void ProcessDraw<std::integral_constant<bool, false>, std::integral_constant<bool, false>, std::integral_constant<bool, false>, std::integral_constant<bool, false>, std::integral_constant<bool, false>, std::integral_constant<bool, true> >(SWR_CONTEXT*, DRAW_CONTEXT*, unsigned int, void*) (frontend.cpp:1370)
==9673==    by 0x106C6CB7: WorkOnFifoFE(SWR_CONTEXT*, unsigned int, unsigned int&) (threads.cpp:645)
==9673==    by 0xFF2E3B9: void QueueWork<true>(SWR_CONTEXT*) (api.cpp:214)
==9673==    by 0xFF2D82C: QueueDraw(SWR_CONTEXT*) (api.cpp:243)
==9673==    by 0xFF2C28E: DrawInstanced(void*, PRIMITIVE_TOPOLOGY, unsigned int, unsigned int, unsigned int, unsigned int) (api.cpp:1140)
==9673==    by 0xFF2C355: SwrDrawInstanced(void*, PRIMITIVE_TOPOLOGY, unsigned int, unsigned int, unsigned int, unsigned int) (api.cpp:1186)
==9673==    by 0xFF0B181: swr_draw_vbo(pipe_context*, pipe_draw_info const*) (swr_draw.cpp:212)
==9673==    by 0x9E95553: cso_draw_vbo (cso_context.c:1670)
==9673==    by 0x9CC704E: st_draw_vbo (st_draw.c:276)

Perhaps in combination with valgrind + gdbserver you can get a workable stack analysis.