If a uniform has an initializer it will now be given as the optional
initializer operand to the OpVariable instruction.

Fixes: https://github.com/KhronosGroup/glslang/issues/1259

Signed-off-by: Neil Roberts <nroberts@igalia.com> (the code)
Signed-off-by: Alejandro Piñeiro <apinheiro@igalia.com> (the tests)
Signed-off-by: Arcady Goldmints-Orlov <agoldmints@igalia.com>
Co-authored-by: Neil Roberts <nroberts@igalia.com>