spirv: Fix InterpolateAt* instructions for vecs with dynamic index

If the glsl is something like this:

  in vec4 some_input;
  interpolateAtCentroid(some_input[idx])

then it now gets generated as if it were:

  interpolateAtCentroid(some_input)[idx]

This is necessary because the index will get generated as a series of
nir_bcsel instructions so it would no longer be an input variable. It
is similar to what is done for GLSL in ca63a5ed3e9efb2bd645b42.

Although I can’t find anything explicit in the Vulkan specs to say
this should be allowed, the SPIR-V spec just says “the operand
interpolant must be a pointer to the Input Storage Class”, which I
guess doesn’t rule out any type of pointer to an input.

This was found using the spec/glsl-4.40/execution/fs-interpolateAt*
Piglit tests with the ARB_gl_spirv branch.

Signed-off-by: Neil Roberts <[email protected]>
Signed-off-by: Alejandro Piñeiro <[email protected]>

v2: update after nir_deref_instr land on master. Implemented by
    Alejandro Piñeiro. Special thanks to Jason Ekstrand for guidance
    at the new nir_deref_instr world.

Reviewed-by: Jason Ekstrand <[email protected]>

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