Assembler: added support for large constants for 2-reg variants (macros)
of 3-reg instructions (add, addu, and, or, slt, sltu, sub, subu, xor).
When mode ".set at" enabled, an additional LI instruction is inserted
sing $1 as a scratch register.
Assembler: mode ".set at" required to enable wide immediates in 2-red macros.
Range check fixed in assembler.