re PR tree-optimization/92645 (Hand written vector code is 450 times slower when compiled with GCC compared to Clang)

2019-11-28  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/92645
	* tree-ssa-forwprop.c (get_bit_field_ref_def): Also handle
	conversions inside a mode class.  Remove restriction on
	preserving the element size.
	(simplify_vector_constructor): Deal with the above and for
	identity permutes also try using VEC_UNPACK_[FLOAT_]LO_EXPR
	and VEC_PACK_TRUNC_EXPR.

	* gcc.target/i386/pr92645-4.c: New testcase.

From-SVN: r278806