backprop: Fix deleting of a phi node [PR116922]

backprop: Fix deleting of a phi node [PR116922]

The problem here is remove_unused_var is called on a name that is
defined by a phi node but it deletes it like removing a normal statement.
remove_phi_node should be called rather than gsi_remove for phinodes.

Note there is a possibility of using simple_dce_from_worklist instead
but that is for another day.

Bootstrapped and tested on x86_64-linux-gnu.

	PR tree-optimization/116922

gcc/ChangeLog:

	* gimple-ssa-backprop.cc (remove_unused_var): Handle phi
	nodes correctly.

gcc/testsuite/ChangeLog:

	* gcc.dg/torture/pr116922.c: New test.

Signed-off-by: Andrew Pinski <quic_apinski@quicinc.com>
(cherry picked from commit cea87c84eacdb422caeada734ba5138c994d7022)