middle-end: don't keep .MEM guard nodes for PHI nodes who dominate loop [PR111860]

middle-end: don't keep .MEM guard nodes for PHI nodes who dominate loop [PR111860]

The previous patch tried to remove PHI nodes that dominated the first loop,
however the correct fix is to only remove .MEM nodes.

This patch thus makes the condition a bit stricter and only tries to remove
MEM phi nodes.

I couldn't figure out a way to easily determine if a particular PHI is vUSE
related, so the patch does:

1. check if the definition is a vDEF and not defined in main loop.
2. check if the definition is a PHI and not defined in main loop.
3. check if the definition is a default definition.

For no 2 and 3 we may misidentify the PHI, in both cases the value is defined
outside of the loop version block which also makes it ok to remove.

gcc/ChangeLog:

	PR tree-optimization/111860
	* tree-vect-loop-manip.cc (slpeel_tree_duplicate_loop_to_edge_cfg):
	Drop .MEM nodes only.

gcc/testsuite/ChangeLog:

	PR tree-optimization/111860
	* gcc.dg/vect/pr111860-2.c: New test.
	* gcc.dg/vect/pr111860-3.c: New test.