Improve CONSTANTLY on constant argument

[Paul F. Dietz]

(moved from a comment in 1852475)

Add a deftransform (or compiler macro?) for CONSTANTLY to optimize the case of a constant argument.

(declaim (optimize (speed 3) (safety 0) (debug 0)))

* (defun f () (lambda (&rest args) (declare (ignore args)) t))
F
* (defun g () (constantly t))
G
* (disassemble 'f)
; disassembly for F
; Size: 13 bytes. Origin: #x52D489A6 ; F
; A6: 488B15D3FFFFFF MOV RDX, [RIP-45] ; #<FUNCTION (LAMBDA
                                                              ; ( ..))>
; AD: 488BE5 MOV RSP, RBP
; B0: F8 CLC
; B1: 5D POP RBP
; B2: C3 RET
NIL
* (disassemble (f))
; disassembly for (LAMBDA (&REST ARGS) :IN F)
; Size: 11 bytes. Origin: #x52D489D6 ; (LAMBDA
                                                                    (
                                                                     &REST
                                                                     ARGS)
                                                                  :IN
                                                                  F)
; D6: BA4F001050 MOV EDX, #x5010004F ; T
; DB: 488BE5 MOV RSP, RBP
; DE: F8 CLC
; DF: 5D POP RBP
; E0: C3 RET
NIL
* (disassemble 'g)
; disassembly for G
; Size: 20 bytes. Origin: #x52D48A56 ; G
; 56: BA4F001050 MOV EDX, #x5010004F ; T
; 5B: B902000000 MOV ECX, 2
; 60: FF7508 PUSH QWORD PTR [RBP+8]
; 63: B8C21A4450 MOV EAX, #x50441AC2 ; #<FDEFN CONSTANTLY>
; 68: FFE0 JMP RAX
NIL
* (disassemble (g))
; disassembly for (LAMBDA (&REST SB-IMPL::ARGUMENTS) :IN CONSTANTLY)
; Size: 17 bytes. Origin: #x5216BB7A ; (LAMBDA
                                                                    (
                                                                     &REST
                                                                     SB-IMPL::ARGUMENTS)
                                                                  :IN
                                                                  CONSTANTLY)
; 7A: 488BE5 MOV RSP, RBP
; 7D: F8 CLC
; 7E: 5D POP RBP
; 7F: C3 RET
; 80: CC10 INT3 16 ; Invalid argument count trap
; 82: 6A20 PUSH 32
; 84: E89747F9FF CALL #x52100320 ; ALLOC-TRAMP-R11
; 89: EB9E JMP #x5216BB29
NIL
*

Most of the uses of CONSTANTLY in the sbcl source are on a constant argument.