i965/vs: Implement vec4_visitor::visit(ir_texture *).

i965/vs: Implement vec4_visitor::visit(ir_texture *).

This translates the GLSL compiler's IR into vec4_instruction IR,
generating code to load coordinates, LOD info, shadow comparitors, and
so on into the appropriate message registers.

It turns out that the SIMD4x2 parameters are identical on Gen 5-7, and
the Gen4 code is similar enough that, unlike in the FS, it's easy enough
to support all generations in a single function.

v2: Load zeros for missing coordinates (fixing vs-texelFetch-sampler1D
and 2D on G45), and fix G45 message length for shadow comparisons.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>