[perl #130855] [SPESH][OPTIMIZER] glitch with anon vars causes incorrect `Cannot assign to an immutable value` error

[via RT]

# New Ticket Created by  Zoffix Znet 
# Please include the string:  [perl #130855]
# in the subject line of all future correspondence about this issue. 
# <URL: https://rt.perl.org/Ticket/Display.html?id=130855 >


Expected no errors, but in the first case it complains about assignment to 
immutable value even though none exist in the code.

    zoffix@VirtualBox:~$ perl6 -e 'm: sub foo () {$ = 42}; for ^2000000 { $ = 
foo }; say now - INIT now'
    Cannot assign to an immutable value
      in block <unit> at -e line 1

    zoffix@VirtualBox:~$ MVM_SPESH_DISABLE=1 perl6 -e 'm: sub foo () {$ = 42}; 
for ^2000000 { $ = foo }; say now - INIT now'
    2.6287172
    zoffix@VirtualBox:~$ perl6 --optimize=off -e 'm: sub foo () {$ = 42}; for 
^2000000 { $ = foo }; say now - INIT now'                                       
                                                                                
                     
    1.3881109
    zoffix@VirtualBox:~$ perl6 -e 'm: sub foo () {$ = 42}; for ^2 { $ = foo }; 
say now - INIT now'                                                             
                                                                                
                    
    0.00196070
    zoffix@VirtualBox:~$