tree-optimization/120358 - bogus PTA with structure access

tree-optimization/120358 - bogus PTA with structure access

When we compute the constraint for something like
MEM[(const struct QStringView &)&tok2 + 32] we go and compute
what (const struct QStringView &)&tok2 + 32 points to and then
add subvariables to its dereference that possibly fall in the
range of the access according to the original refs size.  In
doing that we disregarded that the subvariable the starting
address points to might not be aligned to it and thus the
access might start at any point within that variable.  The following
conservatively adjusts the pruning of adjacent sub-variables to
honor this.

	PR tree-optimization/120358
	* tree-ssa-structalias.cc (get_constraint_for_1): Adjust
	pruning of sub-variables according to the imprecise
	known start offset.