refactor pp_list

refactor pp_list

-move PL_stack_sp and PL_stack_base reads into the branch in which they
 are used, this also removes 1 var from being saved across the function
 call in GIMME, which removes saving and restoring 1 non-vol register
-write SP to PL_stack_sp (PUTBACK) only if it was changed
-POPMARK is mutable, it must execute on all branches

this reduced pp_list's machine code size of the function from 0x58 to
0x53 bytes on VC 2003 -01 32 bits