why is it not done in the compiler

Is it that these things that logically belong in the compiler is moved to the optimizer for no other reason than to make the optimizer more impressive?

if anyone can inform me why e.g.

0000 AF00        R     MOV     R7,b
0002 0500        R     INC     b
0004 A807              MOV     R0,AR7
0006 E6                MOV     A,@R0

nnstead of
0000 A800        R     MOV     R0,b
0002 E6                MOV     A,@R0

can not be done even easier in the compiler than the optimizer, I would LOVE to learn why

Erik