Fix local.set and local.tee types for SIMD

Because Wasm SIMD vectors store their type as `v128`, there is a mismatch between the more specific types Cranelift uses and Wasm SIMD. Because of this mismatch, Wasm SIMD translates to the default Cranelift type `I8X16`, causing issues when more specific type information is available (e.g. `I32x4`). To fix this, all incoming values to SIMD instructions are checked during translation (not runtime) and if necessary cast from `I8X16` to the appropriate type by functions like `optionally_bitcast_vector`, `pop1_with_bitcast` and `pop2_with_bitcast`. However, there are times when we must also cast to `I8X16` for outgoing values, as with `local.set` and `local.tee`.

There are other ways of resolving this (e.g., see adding a new vector type, https://github.com/bytecodealliance/cranelift/pull/1251) but we discussed staying with this casting approach in https://github.com/bytecodealliance/wasmtime/issues/1147.

cranelift/wasmtests/simd.wat

@@ -17,6 +17,12 @@
     i32x4.extract_lane 3
   )
 
+  (func $test_locals (local i32 v128)
+    local.get 0
+    i32x4.splat
+    local.set 1
+  )
+
   (export "test_splat" (func $test_splat))
   (export "test_insert_lane" (func $test_insert_lane))
   (export "test_const" (func $test_const))