d: Fix ICE on named continue label in an unrolled loop [PR107592]

d: Fix ICE on named continue label in an unrolled loop [PR107592]

Continue labels in an unrolled loop require a unique label per
iteration.  Previously this used the Statement body node for each
unrolled iteration to generate a new entry in the label hash table.
This does not work when the continue label has an identifier, as said
named label is pointing to the outer UnrolledLoopStatement node.

What would happen is that during the lowering of `continue label', an
automatic label associated with the unrolled loop would be generated,
and a jump to that label inserted, but because it was never pushed by
the visitor for the loop itself, it subsequently never gets emitted.

To fix, correctly use the UnrolledLoopStatement as the key to look up
and store the break/continue label pair, but remove the continue label
from the value entry after every loop to force a new label to be
generated by the next call to `push_continue_label'

	PR d/107592

gcc/d/ChangeLog:

	* toir.cc (IRVisitor::push_unrolled_continue_label): New method.
	(IRVisitor::pop_unrolled_continue_label): New method.
	(IRVisitor::visit (UnrolledLoopStatement *)): Use them instead of
	push_continue_label and pop_continue_label.

gcc/testsuite/ChangeLog:

	* gdc.dg/pr107592.d: New test.