radeonsi: add a workaround for bitfield_extract when count is 0

This ports the fix from 3d417577. Both LLVM 7 & 8 continue
to have this problem.

It fixes rendering issues in some menu and loading screens of
Civ VI which can be seen in the trace from bug 104602.

Note: This does not fix the black triangles on Vega for bug
104602.

Cc: mesa-stable@lists.freedesktop.org
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=104602
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=107276