[LoopDeletion] Consider infinite loops alive, unless mustprogress.

The current loop or any of its sub-loops may be infinite. Unless the
function or the loops are marked as mustprogress, this in itself makes
the loop *not* dead.

This patch moves the logic to check whether the current loop is finite
or mustprogress to `isLoopDead` and also extends it to check the
sub-loops. This should fix PR50511.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D103382