c++: Fix compile-time-hog in cp_fold_immediate_r [PR111660]

c++: Fix compile-time-hog in cp_fold_immediate_r [PR111660]

My recent patch introducing cp_fold_immediate_r caused exponential
compile time with nested COND_EXPRs.  The problem is that the COND_EXPR
case recursively walks the arms of a COND_EXPR, but after processing
both arms it doesn't end the walk; it proceeds to walk the
sub-expressions of the outermost COND_EXPR, triggering again walking
the arms of the nested COND_EXPR, and so on.  This patch brings the
compile time down to about 0m0.030s.

The ff_fold_immediate flag is unused after this patch but since I'm
using it in the P2564 patch, I'm not removing it now.  Maybe at_eof
can be used instead and then we can remove ff_fold_immediate.

	PR c++/111660

gcc/cp/ChangeLog:

	* cp-gimplify.cc (cp_fold_immediate_r) <case COND_EXPR>: Don't
	handle it here.
	(cp_fold_r): Handle COND_EXPR here.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/hog1.C: New test.
	* g++.dg/cpp2a/consteval36.C: New test.