Small speedup for recently added cprop code

[Jeffrey A Law <law at redhat dot com>]

This patch avoids some useless work in the recently added phi cprop
code.

First, it's insanely stupid to call fold_stmt_in_place on PHI nodes
and marginally stupid to call mark_new_vars_to_rename on PHI nodes.
In fact, for PHIs, all we want/need to do is propagate the value,
then mark the PHI node for later processing to see if it has turned
into a degenerate PHI.

For tramp3d most of our propagations are into PHIs, so this ultimately
results in a lot fewer calls into fold_stmt_in_place.

Second, it's rather dumb to call into the  mark_new_vars_to_rename or
fold_stmt_in_place when we're replacing one virtual SSA_NAME with
another SSA_NAME and the two SSA_NAMEs have the same underlying _DECL
node.  In this case folding will never do anything useful and 
no vars will be exposed/removed from the statement.

The net result of these two changes in roughly a 90% reduction in
calls to fold_stmt_in_place (which calls walk_tree!) and 
mark_new_vars_to_rename from the cprop code.  Sadly, this is only
showing a teeny tiny compile-time improvement, small enough that it
could just be noise.

Bootstrapped and regression tested on i686-pc-linux-gnu.

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