Refactor wi::bswap as a function (instead of a method).

This patch implements Richard Sandiford's suggestion from
https://gcc.gnu.org/pipermail/gcc-patches/2023-May/618215.html
that wi::bswap (and a new wi::bitreverse) should be functions,
and ideally only accessors are member functions.  This patch
implements the first step, moving/refactoring wi::bswap.

2023-05-31  Roger Sayle  <roger@nextmovesoftware.com>
	    Richard Sandiford  <richard.sandiford@arm.com>

gcc/ChangeLog
	* fold-const-call.cc (fold_const_call_ss) <CFN_BUILT_IN_BSWAP*>:
	Update call to wi::bswap.
	* simplify-rtx.cc (simplify_const_unary_operation) <case BSWAP>:
	Update call to wi::bswap.
	* tree-ssa-ccp.cc (evaluate_stmt) <case BUILT_IN_BSWAP*>:
	Update calls to wi::bswap.

	* wide-int.cc (wide_int_storage::bswap): Remove/rename to...
	(wi::bswap_large): New function, with revised API.
	* wide-int.h (wi::bswap): New (template) function prototype.
	(wide_int_storage::bswap): Remove method.
	(sext_large, zext_large): Consistent indentation/line wrapping.
	(bswap_large): Prototype helper function containing implementation.
	(wi::bswap): New template wrapper around bswap_large.