Xinu7/src/cmd/cc11/c2fix

This change fixes a bug in C optimizer.
The file is /usr/src/cmd/c/c21.c.
This is in routine rmove, after case CBR, the first two
lines in the 'if r==0' clause:

190,191c
< 				p->back->back->forw = p->forw;
< 				p->forw->back = p->back->back;
---
> 				if (p->forw->op==CBR
> 				  || p->forw->op==SXT
> 				  || p->forw->op==CFCC) {
> 					p->back->forw = p->forw;
> 					p->forw->back = p->back;
> 				} else {
> 					p->back->back->forw = p->forw;
> 					p->forw->back = p->back->back;
> 				}

The old code deleted a test or compare with constant operands
and a following conditional branch that would always fail.
The new code only deletes the branch (leaves the test)
if the combination is followed by another instruction that
needs the condition codes.  The test and second branch are liable
to be deleted later.