About the unnamed insn defined in the machine.md

[redriver jiang]

    Hello, recently I am porting the GCC backend to a DSP.  The GCC
Internals document says that the unnamed insns are used to translate
RTL insns  to the assembler insns, but I find that the unnamed insn
patterns can be used in combine phase to combine insns by reading the
"*arith_shiftsi" insn pattern in arm.md file..This "*arith_shiftsi"
insn pattern is used to combine the "plus insn" and "shift insn" into
"add rd,Rn,rm,shift immediate_operand".Is my understanding correct?