compiler/spirv: set is_shadow for depth comparitor sampling opcodes

From the SPIR-V spec, OpTypeImage:

"Depth is whether or not this image is a depth image. (Note that
 whether or not depth comparisons are actually done is a property of
 the sampling opcode, not of this type declaration.)"

The sampling opcodes that specify depth comparisons are
OpImageSample{Proj}Dref{Explicit,Implicit}Lod, so we should set
is_shadow only for these (we were using the deph property of the
image until now).

v2:
 - Do the same for OpImageDrefGather.
 - Set is_shadow to false if the sampling opcode is not one of these (Jason)
 - Reuse an existing switch statement instead of adding a new one (Jason)

Fixes crashes in:
dEQP-VK.spirv_assembly.instruction.graphics.image_sampler.depth_property.*

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Cc: mesa-stable@lists.freedesktop.org
(cherry picked from commit 41ac0b14)