middle-end: Skip initialization of opaque type variables [PR103127]

For -ftrivial-auto-var-init=*, skip initializing the variable if it is an
opaque type, because CONST0_RTX(mode) is not defined for opaque modes.

2021-12-01  Peter Bergner  <bergner@linux.ibm.com>

gcc/
	PR middle-end/103127
	* gimplify.c (is_var_need_auto_init): Handle opaque types.

gcc/testsuite/
	PR middle-end/103127
	* gcc.target/powerpc/pr103127.c: New test.