SPIRVReader crashes when translating OpVariable with OpConstantSampler initializer

Below SPIRV code crashes in SPIRV Reader:
```
%2 = OpTypeSampler
%_ptr_UniformConstant_2 = OpTypePointer UniformConstant %2

%_sampler_init = OpConstantSampler %2 Clamp 0 Nearest
%image_sampler = OpVariable %_ptr_UniformConstant_2 UniformConstant %_sampler_init
```
The crash occurs during translation of `OpVariable`s initializer (`%_sampler_init`):
1. `%image_sampler` is translated to llvm `GlobalVariable`, since it's `OpVariable`:
```llvm
@image_sampler = external addrspace(2) constant %spirv.Sampler addrspace(2)*
```
2. `%_sampler_init` is `OpConstantSampler`, so it should be translated to:
```llvm
call %spirv.Sampler addrspace(2)* @__translate_sampler_initializer(i32 23)
```
But translation crashes because call instruction needs a `BasicBlock` which is `null`, because it's passed from `OpVariable` handling which operates globally.

I'm wondering how such case should be handled. The main problem is that `GlobalVariable` initializer must be `Constant`, but it actually isn't `Constant`, because `OpConstantSampler` is translated to `CallInst`.

Start with the SPIRVReader paths for OpVariable and OpConstantSampler described in the report, and trace how the initializer is translated when no BasicBlock is available. Decide how this non-constant initializer should be represented without constructing a call in global scope. Done means the supplied SPIR-V no longer crashes and produces valid LLVM IR.

Written by the indexing model from the issue text.