Simplify maybe_fold_reference API

This simplifies the maybe_fold_reference API reflecting that it
no longer canonicalizes refs (that's done with another function)
but only performs constant folding and thus does nothing for is_lhs.

This in turn allows to rip out quite some dead code and one user
of valid_gimple_rhs_p.

2021-04-16  Richard Biener  <rguenther@suse.de>

	* gimple-fold.c (maybe_fold_reference): Remove is_lhs
	parameter (and assume it to be false).
	(fold_gimple_assign): Adjust, remove all callers of
	maybe_fold_reference calling it with is_lhs true.
	(gimple_fold_call): Likewise.
	(fold_stmt_1): Likewise.