gallivm: do some hack heuristic to disable texture functions

We've seen some cases where performance can hurt quite a bit.
Technically, the more simple the function the more overhead there is
for using a function for this (and the less benefits this provides).
Hence don't do this if we expect the generated code to be simple.
There's an even more important reason why this hurts performance,
which is shaders reusing the same unit with some of the same inputs,
as llvm cannot figure out the calculations are the same if they
are performned in the function (even just reusing the same unit without
any input being the same provides such optimization opportunities though
not very much). This is something which would need to be handled by IPO
passes however.

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