rtl-optimization: Fix uninitialized use of opaque mode variable ICE [PR98872]

rtl-optimization: Fix uninitialized use of opaque mode variable ICE [PR98872]

The initialize_uninitialized_regs function emits (set (reg:) (CONST0_RTX))
for all uninitialized pseudo uses.  However, some modes (eg, opaque modes)
may not have a CONST0_RTX defined, leading to an ICE when we try and create
the initialization insn.  The fix is to skip emitting the initialization
if there is no CONST0_RTX defined for the mode.

2021-02-15  Peter Bergner  <bergner@linux.ibm.com>

gcc/
	PR rtl-optimization/98872
	* init-regs.c (initialize_uninitialized_regs): Skip initialization
	if CONST0_RTX is NULL.

gcc/testsuite/
	PR rtl-optimization/98872
	* gcc.target/powerpc/pr98872.c: New test.