gallivm: handle texel swizzles correctly for d3d10-style sample opcodes

gallivm: handle texel swizzles correctly for d3d10-style sample opcodes

unlike OpenGL, the texel swizzle is embedded in the instruction, so honor
that.
(Technically we now execute both the sampler_view swizzle and the
per-instruction swizzle but this should be quite ok.)

v2: add documentation note as it's not obvious.

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