Sounds to me like you're simply running out of (virtual) memory.
Those allocations in softpipe are quite a waste of memory, albeit not THAT big
(each tile is 32x32 pixels, at 16 bytes, so 16kB, and there's 16 tiles per
shader type and per sampler view - with 6 shader types and 32 sampler views
that adds up to (typically mostly never used...) 48MB.
That's probably also the reason why the map fails with radeonsi.
I have no idea though where all your virtual memory address space went, but I
don't see any evidence it's really related to mesa, albeit there recently was a
new and already fixed memory leak.