Add a return_at_end setting.

The flag guarantees that the generated function does not have any
internal return instructions. If the function returns at all, the return
must be the last instruction.

For now just implement a verifier check for this property. When we get
CFG simplifiers and block layout optimizations, they will need to heed
the flag.

cranelift/filetests/verifier/return_at_end.cton

@@ -0,0 +1,26 @@
+test verifier
+set return_at_end
+
+; The verifier doesn't have an API for verifying with flags without also
+; setting an ISA.
+isa riscv
+
+function %ok(i32) {
+ebb0(v0: i32):
+    brnz v0, ebb1
+    trap
+
+ebb1:
+    trapz v0
+    return
+}
+
+function %bad(i32) {
+ebb0(v0: i32):
+    brnz v0, ebb1
+    return ; error: Internal return not allowed
+
+ebb1:
+    trapz v0
+    return
+}