Fix ICE on view conversion between struct and integer

Fix ICE on view conversion between struct and integer

This happens from prepare_gimple_addressable for the variable to be marked
with DECL_NOT_GIMPLE_REG_P when its initialization is gimplified, so it's
apparently just a matter of setting the flag earlier.

gcc/
	* gimplify.cc (lookup_tmp_var): Add NOT_GIMPLE_REG boolean parameter
	and set DECL_NOT_GIMPLE_REG_P on the variable according to it.
	(internal_get_tmp_var): Add NOT_GIMPLE_REG boolean parameter and
	pass it in the call to lookup_tmp_var.
	(get_formal_tmp_var): Pass false in the call to lookup_tmp_var.
	(get_initialized_tmp_var): Likewise.
	(prepare_gimple_addressable): Call internal_get_tmp_var instead of
	get_initialized_tmp_var with NOT_GIMPLE_REG set to true.

gcc/testsuite/
	* gnat.dg/opt98.ads, gnat.dg/opt98.adb: New test.