tree-optimization/116609 - SLP live lane vectorization with partial vectors

The following implements the simple case of single-lane SLP when
using partial vectors which can use the VEC_EXTRACT_LAST code
generation without changes.  I'll keep the PR open for further
enhancements.

This avoids FAILs of gcc.target/aarch64/sve/live_1.c when using
single-lane SLP for non-grouped stores.

	PR tree-optimization/116609
	* tree-vect-loop.cc (vectorizable_live_operation_1): Support
	partial vectors for single-lane SLP.