Apply pattern initialization only when have_insn_for return true.

Apply pattern initialization only when have_insn_for return true.

For -ftrivial-auto-var-init=pattern, initialize the variable with patterns only
when have_insn_for (SET, mode) return true.  Otherwise initialize it with zeros.
with this change, _Complex long double on X86 is initialized to zero for
pattern initialization.

gcc/ChangeLog:

2021-11-10  qing zhao  <qing.zhao@oracle.com>

	* internal-fn.c (expand_DEFERRED_INIT): Apply pattern initialization
	only when have_insn_for return true for the mode. Fix a memory leak.

gcc/testsuite/ChangeLog:

2021-11-10  qing zhao  <qing.zhao@oracle.com>

	* gcc.target/i386/auto-init-6.c: _Complex long double is initialized
	to zero now with -ftrivial-auto-var-init=pattern.