Non-constant initializers not yet implemented.

Tests: dEQP-VK.spirv_assembly.instruction.graphics.variable_init.*
Tests: dEQP-VK.spirv_assembly.instruction.compute.variable_init.*
Tests: dEQP-VK.spirv_assembly.instruction.compute.opunreachable.*
Tests: dEQP-VK.spirv_assembly.instruction.compute.shader_default_output.int.initialized

Bug: b/131681817
Change-Id: I8b283042d3be15028c2efe10988d6847c6256811
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/30213Reviewed-by: Chris Forbes <chrisforbes@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Presubmit-Ready: Ben Clayton <bclayton@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Tested-by: Ben Clayton <bclayton@google.com>