middle-end/100509 - avoid folding constant to aggregate type

When folding a constant initializer looking through aliases to
incompatible types can lead to us trying to fold a constant
to an aggregate type which can't work.  Simply avoid trying
to constant fold non-register typed symbols.

2021-05-11  Richard Biener  <rguenther@suse.de>

	PR middle-end/100509
	* gimple-fold.c (fold_gimple_assign): Only call
	get_symbol_constant_value on register type symbols.

	* gcc.dg/pr100509.c: New testcase.