llvmpipe: lower mul 2x32_64

Fixes:
dEQP-VK.glsl.builtin.function.integer.umulextended*
dEQP-VK.glsl.builtin.function.integer.imulextended*

Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6339>
1 file changed