Accept redundant alias definitions.

In the text format, allow aliases to be defined multiple times, as long
as they're always aliasing the same value.

write.rs is already emitting redundant aliases, because it emits them at
their uses, so this change allows the parser to be able to parse such
code.

cranelift/filetests/parser/alias.cton

@@ -0,0 +1,32 @@
+test cat
+test verifier
+
+function %basic(i32, i32) -> i32 {
+ebb0(v0: i32, v1: i32):
+    v2 -> v0
+    v3 -> v1
+    v4 = iadd.i32 v2, v3
+    return v4
+}
+
+function %transitive() -> i32 {
+ebb0:
+    v0 = iconst.i32 0
+    v1 -> v0
+    v2 -> v1
+    v3 -> v2
+    v4 -> v3
+    return v4
+}
+
+function %duplicate(i32, i32) -> i32 {
+ebb0(v0: i32, v1: i32):
+    v2 -> v0
+    v2 -> v0
+    v2 -> v0
+    v3 -> v1
+    v3 -> v1
+    v3 -> v1
+    v4 = iadd.i32 v2, v3
+    return v4
+}