In disassembly, I see instruction LI, but I can't find such instruction in MIPS instruction set
Oh, I really missed that I use -mips16 compiler option. So, LI instruction is definitely from there (here's MIPS16e docs)

As to that la, it seems to be Assembler pseudo-instruction, which is specified in MIPS32 Instruction Set Quick Reference.

Thanks to guys from Microchip forum for help.