Deprecate libxsmm_x86_instruction_full_vec_load_of_constants

[FreddieWitherden]

It looks as if several parts of libxsmm still use the libxsmm_x86_instruction_full_vec_load_of_constants method. This is a bit of an awkward function and should probably be replaced by the constant data support. (I added this a while back for the asparse_reg code and used it to migrated away from the constant loading; It has several advantages including avoiding a jump, reduced LI$ usage, and deduplication.)