gallivm: zero init the temporary register storage.

Due to flow control we can end up with random values in here having
side effects.

This fixes a crash in gtk4-demo.

Fixes: 44a6b0107b37 ("gallivm: add nir->llvm translation (v2)")
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7327>
1 file changed