completely redo handling of SCONV nodes to cope with Tahoe's

completely redo handling of SCONV nodes to cope with Tahoe's
incorrect handling of char's and short's in registers (e.g. cvtwl r0,r0
does a movl); also correct OPLOG handling for items shorter than 32-bits:
tst[wb] on registers don't work, but cmp[bw] $0,r? does; finally, handle
change to local.c which leaves in lots of new SCONV's (we optimize those
which are extraneous in code added mainly to optim2)

SCCS-vsn: old/pcc/ccom.tahoe/local2.c 1.8
SCCS-vsn: old/pcc/ccom.tahoe/table.c 1.5