Sema: fix analyzeBlockBody logic

Previously, when a coercion needed to be inserted into a break
instruction, the `br` AIR instruction would be rewritten so that the
block operand was a sub-block that did the coercion. The problem is that
the sub-block itself was never added to the parent block, resulting in
the `br` instruction operand being a bad reference.

Now, the `br` AIR instruction that needs to have coercion instructions
added is replaced with the sub-block itself with type `noreturn`, and
then the sub-block has the coercion instructions and a new `br`
instruction that breaks from the original block.

LLVM backend needed to be fixed to lower `noreturn` blocks without
emitting an unused LLVM basic block.

test/behavior/optional.zig

@@ -136,3 +136,26 @@ test "unwrap function call with optional pointer return value" {
     try S.entry();
     comptime try S.entry();
 }
+
+test "nested orelse" {
+    const S = struct {
+        fn entry() !void {
+            try expect(func() == null);
+        }
+        fn maybe() ?Foo {
+            return null;
+        }
+        fn func() ?Foo {
+            const x = maybe() orelse
+                maybe() orelse
+                return null;
+            _ = x;
+            unreachable;
+        }
+        const Foo = struct {
+            field: i32,
+        };
+    };
+    try S.entry();
+    comptime try S.entry();
+}