Do not chain src1 to dest if they should be different types of registers
mips: added _emit() methods for cvt.s.d and trunc.w.d
mips:  and  are temporary registers
mips: implement convdf and convdl operations
mips: added _emit() methods for conversion instructions cvt.s.{d,w}, cvt.d.{s,w} and trunc.w.s
mips: implemented operations convfd, convfl, convld and convlf
mips: implement the convlw operation