Add a new fusion instruction and remove unnecessary ones

[qwe661234]

After constant propagation and constant folding, we can optimize some ALU or brnach instructions into simple instructions.

Close: #226

[jserv]

Consistently employ the prefix constopt_ rather than const_opt to prevent potential confusion.

[jserv]

Both constant folding and constant propagation are related compiler optimization techniques. Constant propagation replaces the bound variable with the constant expression it is bound to. Conversely, constant folding evaluates the expression with all compile-time inputs. I am a bit confused about the proposed changes, as I cannot ensure that a fully functional implementation exists for either constant folding or constant propagation. Can you verify this?

[qwe661234]

Both constant folding and constant propagation are related compiler optimization techniques. Constant propagation replaces the bound variable with the constant expression it is bound to. Conversely, constant folding evaluates the expression with all compile-time inputs. I am a bit confused about the proposed changes, as I cannot ensure that a fully functional implementation exists for either constant folding or constant propagation. Can you verify this?

There is a simple example in ./build/dhrystone.elf can be optimized by constant folding and constant propagation.

• Before

   0x100f0:	auipc	gp,0x18
   0x100f4:	add	gp,gp,864 
   0x100f8:	add	a0,gp,-1996
   0x100fc:	auipc	a2,0x1a
   0x10100:	add	a2,a2,1384 #
   0x10104:	sub	a2,a2,a0

• After

   0x100f0:	lui	gp,  0x10108        
   0x100f4:	lui	gp,  0x10468
   0x100f8:	lui	a0, 0xFC9C
   0x100fc:	lui	a2,0x10116
   0x10100:	lui	a2, 0x1067E 
   0x10104:	lui    a2, 0x9e2

Moreover, these 6 lui instructions can be fused into a fused instruction.

[jserv]

There is a simple example in build/dhrystone.elf can be optimized by constant folding and constant propagation.
[...]
Moreover, these 6 lui instructions can be fused into a fused instruction.

Describe this within git commit messages.