gallivm,tgsi: fix idiv by zero crash

While the result of signed integer division by zero is undefined by glsl
(and doesn't exist with d3d10), we must not crash, so need to make sure we
don't get sigfpe much like udiv already does.
Unlike udiv where we return 0xffffffff (as required by d3d10) there is
no requirement right now to return anything specific so we use zero.

(cherry picked from commit ffeb77c7b0552a8624e46e65d6347240ac5ae84d)
Nominated-by: Roland Scheidegger <sroland@vmware.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=83570