tree-optimization/99863 - clear vector CTOR TREE_SIDE_EFFECTS

tree-optimization/99863 - clear vector CTOR TREE_SIDE_EFFECTS

When we gimplify a vector CTOR the original CONSTRUCTOR tree remains
but we fail to recompute flags such as TREE_SIDE_EFFECTS.  This causes
later GENERIC folding of them in vector lowering to give up since
the match.pd machinery is careful about TREE_SIDE_EFFECTS.

Fixing this makes vector lowering produce much less garbage and
thus following the IL for PR99793 easier.

2021-04-01  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/99863
	* gimplify.c (gimplify_init_constructor): Recompute vector
	constructor flags.