pp_hir: fix printing of swizzles
pp_hir: fix null dereference when deleting
pp_hir: fix bug with conversion from GLSL IR

is_assignee was true upon leaving assignments, wreaking havoc with
branch conditions.
pp_hir: fix using uninitialized data in prog_add_predecessors()

Also, allow the function to be called more than once.
pp_lir: fix mul-add peephole when the add has the same arguments