gallivm: use 2 srcs for 32->16bit conversions in lp_bld_conv_auto

If we only feed one source vector at a time, we cannot use pack intrinsics
(as we only have a 64bit destination dst vector). lp_bld_conv_auto is
specifically designed to alter the length and number of destination vectors,
so this works just fine (if we use single source vectors at a time, afterwards
we immediately reassemble the vectors).
For AVX though this isn't really possible, since we expect 128bit output
already for a single 256bit input. (One day we should handle AVX2 which again
would need multiple inputs, however there's the problem that we get different
ordered output there and we don't want to reorder, so would need to be able
to tell build_conv to handle upper and lower halfs independently.)
A similar strategy would probably work for 32->8bit too (if it doesn't hit
the special case) but I'm going to try something different for that...

Reviewed-by: Jose Fonseca <[email protected]>

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