tree-optimization/117277 - remove CLOBBERs before SLP code generation

tree-optimization/117277 - remove CLOBBERs before SLP code generation

We have to remove CLOBBERs before SLP is code generated since for
store-lanes we are inserting our own CLOBBERs that we want to survive.
So the following refactors vect_transform_loop to remove unwanted
stmts first.

This resolves the gcc.target/aarch64/sve/store_lane_spill_1.c FAIL.

	PR tree-optimization/117277
	* tree-vect-loop.cc (vect_transform_loop): Remove CLOBBERs
	and prefetches before doing any code generation.