gallivm: fix 64->16 f2f16

llvm appears to callout to a library to do 64-bit->16-bit
fp trunc, just trunc to 32-bit first to avoid it.

Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7309>
2 files changed