make better use of LCSSA information in RewriteLoopExitValues. Before, we

would scan the entire loop body, then scan all users of instructions in the
loop, looking for users outside the loop.  Now, since we know that the
loop is in LCSSA form, we know that any users outside the loop will be LCSSA
phi nodes.  Just scan them.

This speeds up indvars significantly.

llvm-svn: 34898