tree-optimization/90579 - avoid STLF fail by better optimizing

tree-optimization/90579 - avoid STLF fail by better optimizing

For the testcase in question which uses a fold-left vectorized
reduction of a reverse iterating loop we'd need two forwprop
invocations to first bypass the permute emitted for the reverse
iterating loop and then to decompose the vector load that only
feeds element extracts.  The following moves the first transform
to a match.pd pattern and makes sure we fold the element extracts
when the vectorizer emits them so the single forwprop pass can
then pick up the vector load decomposition, avoiding the forwarding
fail that causes.

Moving simplify_bitfield_ref also makes forwprop remove the dead
VEC_PERM_EXPR via the simple-dce it uses - this was also
previously missing.

	PR tree-optimization/90579
	* tree-ssa-forwprop.cc (simplify_bitfield_ref): Move to
	match.pd.
	(pass_forwprop::execute): Adjust.
	* match.pd (bit_field_ref (vec_perm ...)): New pattern
	modeled after simplify_bitfield_ref.
	* tree-vect-loop.cc (vect_expand_fold_left): Fold the
	element extract stmt, combining it with the vector def.

	* gcc.target/i386/pr90579.c: New testcase.