Rename logb and significand folds

Rename logb and significand folds

fold_builtin_logb and fold_builtin_significand now only handle
constant arguments, so this patch renames them to fold_const...,
to match fold_const_builtin_pow.  The idea is to differentiate
constant-only folds so that they can be moved to a const_binop-like
function in future.

The functions also had some unnecessary calls to STRIP_NOPS, which
I think are left over from code that has already moved to match.pd.

Tested on x86_64-linux-gnu, aarch64-linux-gnu and arm-linux-gnueabi.

gcc/
	* builtins.c (fold_builtin_logb): Rename to...
	(fold_const_builtin_logb): ...this and remove STRIP_NOPS call.
	(fold_builtin_significand): Rename to...
	(fold_const_builtin_significand): ...this and remove STRIP_NOPS call.
	(fold_builtin_1): Update accordingly.

From-SVN: r229433