tree-optimization/111228 - combine two VEC_PERM_EXPRs

tree-optimization/111228 - combine two VEC_PERM_EXPRs

The following adds simplification of two VEC_PERM_EXPRs where
the later one replaces all elements from either the first or the
second input of the earlier permute.  This allows a three input
permute to be simplified to a two input one.

I'm following the existing two input simplification case and only
allow non-VLA permutes.  The now existing three cases and the
single case in tree-ssa-forwprop.cc somehow ask for merging,
I'm not doing this as part of this change though.

	PR tree-optimization/111228
	* match.pd ((vec_perm (vec_perm ..) @5 ..) -> (vec_perm @x @5 ..)):
	New simplifications.

	* gcc.dg/tree-ssa/forwprop-42.c: New testcase.