Since pres != nil was already checked and the routine sets *pres to false
and returns, the subsequent tests are unneeded.
R=rsc
APPROVED=rsc
DELTA=4 (0 added, 4 deleted, 0 changed)
OCL=33439
CL=33441
introduce NodeList* type in compiler to replace OLIST.
this clarifies where lists can and cannot occur.
list append and concatenation are now cheap.
the _r rules are gone from yacc.
rev and unrev are gone.
no more lists of lists.
the representation of assignments is a bit clunkier.
split into OAS (1=1) and OAS2 (2 or more on one side).
delete dead chanrecv3 code.
delay construction of func types.
R=ken
OCL=31745
CL=31762
