[gimplefe] fix SSA operand creation

[gimplefe] fix SSA operand creation

When transitioning gcc.dg/torture/pr84830.c to a GIMPLE testcase to
feed the IL into PRE that caused the original issue (and verify it's
still there with the fix reverted), I noticed we put up SSA operands
before having fully parsed the function and thus with not all
variables having the final TREE_ADDRESSABLE state.  The following
fixes this, delaying update_stmt calls to when we create PHI nodes.
It also makes the pr84830.c not rely on the particular fake exit
edge source location by making the loop have an exit.

gcc/c/
	* gimple-parser.cc (c_parser_parse_gimple_body): Initialize
	SSA operands for each stmt.
	(c_parser_gimple_compound_statement): Append stmts without
	updating SSA operands.

gcc/testsuite/
	* gcc.dg/torture/pr84830.c: Turn into GIMPLE unit test for PRE.