handle Type_Register in redundant slices.

[katrinafyi]

sp is given a "register" type and this wasn't handled by the redundant slice elimination which considered arrays and bitvectors.

$ echo ':sem A64 0xd10083ff' | dune exec asli                                                                                                                                                                                                         
Decoding instruction A64 d10083ff     
SP_EL0 = add_bits.0 {{ 64 }} ( add_bits.0 {{ 64 }} ( SP_EL0,'1111111111111111111111111111111111111111111111111111111111011111' ),'0000000000000000000000000000000000000000000000000000000000000001' ) ;

fixes #41.