Remove the return_reg instruction.

RISC architectures that take a return address in a register can use a
special-purpose `link` return value to do so.

filetests/regalloc/basic.cton

@@ -8,5 +8,5 @@ ebb0(v1: i32, v2: i32):
     v3 = iadd v1, v2
 ; check: [R#0c,%x0]
 ; sameln: iadd
-    return_reg v3
+    return
 }