avoid hardreg autoinit

This avoids initializating "uninitialized" hardregs like SP.

2021-10-04  Richard Biener  <rguenther@suse.de>

	* gimplify.c (is_var_need_auto_init): DECL_HARD_REGISTER
	variables are not to be initialized.

	* gcc.dg/auto-init-hardreg-1.c: New testcase.