forwprop: Ensure that shuffle masks are VECTOR_CSTs

As reported in PR118487, it is possible that the mask parameter
of a __builtin_shuffle() is not a VECTOR_CST.
If this is the case and checking is enabled then an ICE is triggered.
Let's add a check to fix this issue.

	PR tree-optimization/118487

gcc/ChangeLog:

	* tree-ssa-forwprop.cc (recognise_vec_perm_simplify_seq):
	Ensure that shuffle masks are VECTOR_CSTs.

gcc/testsuite/ChangeLog:

	* gcc.dg/tree-ssa/pr118487.c: New test.

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