Optimize nested permutation to single VEC_PERM_EXPR [PR54346]

Optimize nested permutation to single VEC_PERM_EXPR [PR54346]

        This patch implemented the optimization in PR 54346, which Merges

	c = VEC_PERM_EXPR <a, b, VCST0>;
        d = VEC_PERM_EXPR <c, c, VCST1>;
                to
        d = VEC_PERM_EXPR <a, b, NEW_VCST>;

	Bootstrapped and regtested on x86_64-pc-linux-gnu{-m32,}
        tree-ssa/forwprop-19.c fail to pass but I'm not sure whether it
        is ok to removed it.

gcc/ChangeLog:

	PR tree-optimization/54346
	* match.pd: Merge the index of VCST then generates the new vec_perm.

gcc/testsuite/ChangeLog:

	* gcc.dg/pr54346.c: New test.

Co-authored-by: liuhongt <hongtao.liu@intel.com>