PR 102281 (-ftrivial-auto-var-init=zero causes ice)

Do not add call to __builtin_clear_padding when a variable is a gimple
register or it might not have padding.

gcc/ChangeLog:

2021-11-01  qing zhao  <qing.zhao@oracle.com>

	* gimplify.c (gimplify_decl_expr): Do not add call to
	__builtin_clear_padding when a variable is a gimple register
	or it might not have padding.
	(gimplify_init_constructor): Likewise.

gcc/testsuite/ChangeLog:

2021-11-01  qing zhao  <qing.zhao@oracle.com>

	* c-c++-common/pr102281.c: New test.
	* gcc.target/i386/auto-init-2.c: Adjust testing case.
	* gcc.target/i386/auto-init-4.c: Likewise.
	* gcc.target/i386/auto-init-6.c: Likewise.
	* gcc.target/aarch64/auto-init-6.c: Likewise.