Keep virtual SSA up-to-date in vectorizer

The following removes a FIXME where we fail(ed) to keep virtual
SSA up-to-date, patching up the remaining two cases I managed to
trigger.  I've left an assert so that we pick up cases arising
for the cases I wasn't able to trigger.

2022-07-04  Richard Biener  <rguenther@suse.de>

	* tree-vect-loop-manip.cc (vect_do_peeling): Assert that
	no SSA update is needed instead of updating virtual SSA
	form.
	* tree-vect-stmts.cc (vectorizable_load): For hoisted
	invariant load use the loop entry virtual use.
	For emulated gather loads use the virtual use of the
	original stmt like vect_finish_stmt_generation would do.