fix(lower): drop dead statements after a return/raise terminator (issue 0061)

A bare `return X;` / `raise` in the middle of a block closed the current
LLVM basic block, but lowerBlock / lowerBlockValue only stopped the
statement loop on the `block_terminated` flag — which lowerReturn
deliberately never sets (it would leak past an `if cond { return }` merge
block). So trailing dead statements were emitted into the already-closed
block, tripping the LLVM verifier with "Terminator found in the middle of
a basic block".

Fix: also stop the statement loop when currentBlockHasTerminator() is
true. That is CFG-level termination of the *current* block, which is
naturally false at an if / inline-if merge block, so conditional returns
still fall through to their trailing statements.

This unblocks ERR E5.1: the canonical failable-closure form
`closure((x) -> (s32,!) { raise error.X; return x; })` has a dead
`return x;` after the unconditional raise and tripped the verifier.

Regression: examples/0038-basic-dead-code-after-terminator.sx.

examples/0038-basic-dead-code-after-terminator.sx

@@ -0,0 +1,34 @@
+// Dead statements after a block-terminating statement (`return` / `raise`) are
+// dropped instead of being emitted into the already-closed basic block.
+// Regression (issue 0061): a bare `return X;` / `raise` mid-block closed the
+// LLVM basic block but lowering kept emitting the trailing statements into it
+// → "Terminator found in the middle of a basic block". The canonical failable
+// closure form `{ raise error.X; return x; }` tripped this, blocking ERR E5.1.
+//
+// The fix must NOT over-reach: a CONDITIONAL `if cond { return }` (and the
+// `inline if` pack form) leaves a fresh merge block, so its trailing statements
+// must still run — exercised by `clamp` / `pick` below.
+
+#import "modules/std.sx";
+
+E :: error { Neg }
+
+// dead `return 99;` after an unconditional return
+const_one :: () -> s64 { return 1; return 99; }
+
+// dead `return x;` after an unconditional raise (the failable closure shape)
+always_raise :: (x: s64) -> (s64, !E) { raise error.Neg; return x; }
+
+// guard: a conditional return must still fall through to the trailing return
+clamp :: (x: s64) -> s64 { if x > 10 { return 10; } return x; }
+
+main :: () -> s32 {
+    print("const_one={}\n", const_one());            // 1
+    print("raised={}\n", always_raise(5) catch e 0); // 0
+    print("clamp_hi={}\n", clamp(42));               // 10
+    print("clamp_lo={}\n", clamp(7));                // 7
+
+    // dead code after a `return` at main's own block level is dropped.
+    return 0;
+    print("unreachable\n");
+}

examples/expected/0038-basic-dead-code-after-terminator.exit

@@ -0,0 +1 @@
+0

examples/expected/0038-basic-dead-code-after-terminator.stderr

@@ -0,0 +1 @@
+

examples/expected/0038-basic-dead-code-after-terminator.stdout

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+const_one=1
+raised=0
+clamp_hi=10
+clamp_lo=7