Make gimple_could_trap_p const-safe.

Make gimple_could_trap_p const-safe.

Allow gimple_could_trap_p (which previously took a non-const gimple)
to be called from functions that take a const gimple (such as
gimple_has_side_effects), and update its prototypes.  Pre-approved
as obvious.

2021-07-13  Roger Sayle  <roger@nextmovesoftware.com>
	    Richard Biener  <rguenther@suse.de>

gcc/ChangeLog
	* gimple.c (gimple_could_trap_p_1):  Make S argument a
	"const gimple*".  Preserve constness in call to
	gimple_asm_volatile_p.
	(gimple_could_trap_p): Make S argument a "const gimple*".
	* gimple.h (gimple_could_trap_p_1, gimple_could_trap_p):
	Update function prototypes.