tree-optimization/100253 - fix bogus aligned vectorized loads/stores

tree-optimization/100253 - fix bogus aligned vectorized loads/stores

At some point DR_MISALIGNMENT was supposed to be -1 when the
access was not element aligned.  That's obviously not true at this
point so this adjusts both store and load vectorizing to no longer
assume this which in turn allows simplifying the code.

2021-04-29  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/100253
	* tree-vect-stmts.c (vectorizable_load): Do not assume
	element alignment when DR_MISALIGNMENT is -1.
	(vectorizable_store): Likewise.

	* g++.dg/pr100253.C: New testcase.