1 extra bytecode instruction per variable per loop

Hi Bruno,

The fix for https://sourceforge.net/p/clisp/bugs/375/
(https://sourceforge.net/p/clisp/clisp/ci/52d209b7d29ddf1125d2d89f4185c20c2f09da12/)
introduces an extra binding per init expression.
This results in an extra bytecode instruction in the preamble, e.g.,
for (loop repeat 100 for a on b for x in y collect (list a x)):

--8<---------------cut here---------------start------------->8---
Disassembly of function :LAMBDA
(CONST 0) = 100
(CONST 1) = B
(CONST 2) = Y
0 required arguments
0 optional arguments
No rest parameter
No keyword parameters
reads special variables: Y B
27 byte-code instructions:
0     (CONST&PUSH 0)                      ; 100
1     (PUSH-NIL 4)
3     (GETVALUE 1)                        ; B
5     (STORE 3)
6     (GETVALUE 2)                        ; Y
8     (STORE 2)
9     (JMP L26)
11    L11
--8<---------------cut here---------------end--------------->8---

becomes

--8<---------------cut here---------------start------------->8---
Disassembly of function :LAMBDA
(CONST 0) = B
(CONST 1) = Y
(CONST 2) = 100
0 required arguments
0 optional arguments
No rest parameter
No keyword parameters
reads special variables: Y B
29 byte-code instructions:
0     (GETVALUE&PUSH 0)                   ; B
2     (GETVALUE&PUSH 1)                   ; Y
4     (CONST&PUSH 2)                      ; 100
5     (PUSH-NIL 4)
7     (LOAD 6)
8     (STORE 3)
9     (LOAD 5)
10    (STORE 2)
11    (JMP L28)
13    L13
--8<---------------cut here---------------end--------------->8---

The cost is probably trivial, but I wonder if there is some simple
compilation optimization missing here...

Thanks

-- 
Sam Steingold (http://sds.podval.org/) on darwin Ns 10.3.1504
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