Account for duplicated if-block params on end op in unreachable case.

This is a close analogue to bnjbvr@'s fix in commit 518b7a7e. Similar to
that fix, this PR fixes a bug in which the Wasm translator could
misalign its value stack and either mistranslate or cause a panic with a
type-checking error.

Found via fuzzing by :decoder in SpiderMonkey (bug 1664453).

cranelift/wasmtests/if-unreachable-else-params-2.wat

@@ -0,0 +1,18 @@
+(module
+  (type (;0;) (func (param i32 i32) (result f64)))
+  (func $main (type 0) (param i32 i32) (result f64)
+    f64.const 1.0
+    local.get 0
+    local.get 1
+    if (param i32)  ;; label = @2
+      i64.load16_s align=1
+      drop
+    else
+      unreachable
+    end)
+  (table (;0;) 63 255 funcref)
+  (memory (;0;) 13 16)
+  (export "t1" (table 0))
+  (export "m1" (memory 0))
+  (export "main" (func $main))
+  (export "memory" (memory 0)))