compiler: lower func values to switch + direct call

This has several advantages, among them:
  - Many passes (heap-to-stack, dead arg elimination, inlining) do not
    work with function pointer calls. Making them normal function calls
    improves their effectiveness.
  - Goroutine lowering to LLVM coroutines does not currently support
    function pointers. By eliminating function pointers, coroutine
    lowering gets support for them for free.
    This is especially useful for WebAssembly.
Because of the second point, this work is currently only enabled for the
WebAssembly target.

compiler/optimizer.go

@@ -43,6 +43,7 @@ func (c *Compiler) Optimize(optLevel, sizeLevel int, inlinerThreshold uint) erro
 		c.OptimizeStringToBytes()
 		c.OptimizeAllocs()
 		c.LowerInterfaces()
+		c.LowerFuncValues()
 
 		// After interfaces are lowered, there are many more opportunities for
 		// interprocedural optimizations. To get them to work, function
@@ -76,6 +77,7 @@ func (c *Compiler) Optimize(optLevel, sizeLevel int, inlinerThreshold uint) erro
 	} else {
 		// Must be run at any optimization level.
 		c.LowerInterfaces()
+		c.LowerFuncValues()
 		err := c.LowerGoroutines()
 		if err != nil {
 			return err