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>

src/gallium/docs/source/tgsi.rst

@@ -1730,6 +1730,8 @@ Resource Sampling Opcodes
 
 Those opcodes follow very closely semantics of the respective Direct3D
 instructions. If in doubt double check Direct3D documentation.
+Note that the swizzle on SVIEW (src1) determines texel swizzling
+after lookup.
 
 .. opcode:: SAMPLE - Using provided address, sample data from the
                specified texture using the filtering mode identified