[PHIOPT/MATCH] Remove the statement to move if not used

[PHIOPT/MATCH] Remove the statement to move if not used

Instead of waiting for DCE to remove the unused statement,
and maybe optimize another conditional, it is better if
we don't move the statement and have the statement
removed.

OK? Bootstrapped and tested on x86_64-linux-gnu.

Changes from v1:
* v2: Change the order of insertation and check to see if the lhs
  is used rather than see if the lhs was used in the sequence.

gcc/ChangeLog:

	* tree-ssa-phiopt.c (match_simplify_replacement): Move
	insert of the sequence before the movement of the
	statement. Check if to see if the statement is used
	outside of the original phi to see if we should move it.

gcc/testsuite/ChangeLog:

	* gcc.dg/tree-ssa/pr96928-1.c: Update to similar as pr96928.c.