64-bit version of 7425af4b7aaa31da10bd1bc7996d3bb212c79d88. We
still need to lower to 32-bit v_accagpr_write_b32s, so this has
a unique value restriction that requires both halves of the constant
to be 32-bit inline immediates. This only introduces the new
pseudo definitions, but doesn't try to use them yet.
694a488708