Add FuncEnvironment hooks to generate prologue and epilogue code.

In some cases, it is useful to do some work at entry to or exit from a
Cranelift function translated from WebAssembly. This PR adds two
optional methods to the `FuncEnvironment` trait to do just this,
analogous to the pre/post-hooks on operators that already exist.

This PR also includes a drive-by compilation fix due to the latest
nightly wherein `.is_empty()` on a `Range` ambiguously refers to either
the `Range` impl or the `ExactSizeIterator` impl and can't resolve.

cranelift/wasm/src/func_translator.rs

@@ -225,6 +225,7 @@ fn parse_function_body<FE: FuncEnvironment + ?Sized>(
     // The control stack is initialized with a single block representing the whole function.
     debug_assert_eq!(state.control_stack.len(), 1, "State not initialized");
 
+    environ.before_translate_function(builder, state)?;
     while !reader.eof() {
         let pos = reader.original_position();
         builder.set_srcloc(cur_srcloc(&reader));
@@ -234,6 +235,7 @@ fn parse_function_body<FE: FuncEnvironment + ?Sized>(
         translate_operator(validator, &op, builder, state, environ)?;
         environ.after_translate_operator(&op, builder, state)?;
     }
+    environ.after_translate_function(builder, state)?;
     let pos = reader.original_position();
     validator.finish(pos)?;
 
@@ -243,7 +245,6 @@ fn parse_function_body<FE: FuncEnvironment + ?Sized>(
     // If the exit block is unreachable, it may not have the correct arguments, so we would
     // generate a return instruction that doesn't match the signature.
     if state.reachable {
-        debug_assert!(builder.is_pristine());
         if !builder.is_unreachable() {
             match environ.return_mode() {
                 ReturnMode::NormalReturns => {