forwprop: Eliminate redundant calls to to_constant()

When extracting the amount of vector elements, we currently
first check if the value is a contant with is_constant(),
followed by obtaining the value with to_constant(),
which internally calls is_constant() again.
We can address this by using is_constant (T*), which also
provides the constant value.

gcc/ChangeLog:

	* tree-ssa-forwprop.cc (recognise_vec_perm_simplify_seq):
	Eliminate redundant calls to to_constant().

Signed-off-by: Christoph Müllner <christoph.muellner@vrull.eu>