main: add -print-allocs flag that lets you print all heap allocations

This flag, if set, is a regexp for function names. If there are heap
allocations in the matching function names, these heap allocations will
be printed with an explanation why the heap allocation exists (and why
the object can't be stack allocated).

transform/optimizer.go

@@ -3,6 +3,8 @@ package transform
 import (
 	"errors"
 	"fmt"
+	"go/token"
+	"os"
 
 	"github.com/tinygo-org/tinygo/compileopts"
 	"github.com/tinygo-org/tinygo/compiler/ircheck"
@@ -65,7 +67,7 @@ func Optimize(mod llvm.Module, config *compileopts.Config, optLevel, sizeLevel i
 		OptimizeMaps(mod)
 		OptimizeStringToBytes(mod)
 		OptimizeReflectImplements(mod)
-		OptimizeAllocs(mod)
+		OptimizeAllocs(mod, nil, nil)
 		err := LowerInterfaces(mod, sizeLevel)
 		if err != nil {
 			return []error{err}
@@ -86,7 +88,9 @@ func Optimize(mod llvm.Module, config *compileopts.Config, optLevel, sizeLevel i
 		goPasses.Run(mod)
 
 		// Run TinyGo-specific interprocedural optimizations.
-		OptimizeAllocs(mod)
+		OptimizeAllocs(mod, config.Options.PrintAllocs, func(pos token.Position, msg string) {
+			fmt.Fprintln(os.Stderr, pos.String()+": "+msg)
+		})
 		OptimizeStringToBytes(mod)
 		OptimizeStringEqual(mod)