Fold constant offset into memory addressing mode.

[Andrzej Walczak]

Feature request.

Often SBCL(Python) generates this amd64 assembly for addressing memory with base, an index and a constant offset.

; 99: 48D1FF SAR RDI, 1
; 9C: 4805A0000000 ADD RAX, 160
; A2: 48D1F8 SAR RAX, 1
; A5: 0FB61438 MOVZX EDX, BYTE PTR [RAX+RDI]
; A9: 48D1E2 SHL RDX, 1

The 80 (160/2) constant could be folded into the addressing mode like this:

; 99: 48D1FF SAR RDI, 1
; XX: 48D1F8 SAR RAX, 1
; XX: XXXXXXXX MOVZX EDX, BYTE PTR [RAX+RDI+80]
; XX: 48D1E2 SHL RDX, 1