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[OpenCL] Generate opaque type for sampler_t and function call for the…

Description

[OpenCL] Generate opaque type for sampler_t and function call for the initializer

Currently Clang use int32 to represent sampler_t, which have been a source of issue for some backends, because in some backends sampler_t cannot be represented by int32. They have to depend on kernel argument metadata and use IPA to find the sampler arguments and global variables and transform them to target specific sampler type.

This patch uses opaque pointer type opencl.sampler_t* for sampler_t. For each use of file-scope sampler variable, it generates a function call of translate_sampler_initializer. For each initialization of function-scope sampler variable, it generates a function call of translate_sampler_initializer.

Each builtin library can implement its own translate_sampler_initializer(). Since the real sampler type tends to be architecture dependent, allowing it to be initialized by a library function simplifies backend design. A typical implementation of translate_sampler_initializer could be a table lookup of real sampler literal values. Since its argument is always a literal, the returned pointer is known at compile time and easily optimized to finally become some literal values directly put into image read instructions.

This patch is partially based on Alexey Sotkin's work in Khronos Clang (https://github.com/KhronosGroup/SPIR/commit/3d4eec61623502fc306e8c67c9868be2b136e42b).

Differential Revision: https://reviews.llvm.org/D21567