gallivm: fix F2U opcode

gallivm: fix F2U opcode

Previously, we were really doing F2I. And also move it to generic section.
(Note that for llvmpipe the code generated is definitely bad, due to lack
of unsigned conversions with sse. I think though what llvm does (using scalar
conversions to 64bit signed either with x87 fpu (32bit) or sse (64bit)
including lots of domain changes is quite suboptimal, could do something like
is_large = arg >= 2^31
half_arg = 0.5 * arg
small_c = fptoint(arg)
large_c = fptoint(half_arg) << 1
res = select(is_large, large_c, small_c)
which should be much less instructions but that's something llvm should do
itself.)

This fixes piglit fs/vs-float-uint-conversion.shader_test (maybe more, needs
GL 3.0 version override to run.)

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>
Reviewed-by: Zack Rusin <zackr@vmware.com>