tree-optimization/108724 - vectorized code getting piecewise expanded

tree-optimization/108724 - vectorized code getting piecewise expanded

This fixes an oversight to when removing the hard limits on using
generic vectors for the vectorizer to enable both SLP and BB
vectorization to use those.  The vectorizer relies on vector lowering
to expand plus, minus and negate to bit operations but vector
lowering has a hard limit on the minimum number of elements per
work item.  Vectorizer costs for the testcase at hand work out
to vectorize a loop with just two work items per vector and that
causes element wise expansion and spilling.

The fix for now is to re-instantiate the hard limit, matching what
vector lowering does.  For the future the way to go is to emit the
lowered sequence directly from the vectorizer instead.

	PR tree-optimization/108724
	* tree-vect-stmts.cc (vectorizable_operation): Avoid
	using word_mode vectors when vector lowering will
	decompose them to elementwise operations.

	* gcc.target/i386/pr108724.c: New testcase.