pass: ensure frame pointer register is saved (#174)

Currently `avo` uses `BP` as a standard general-purpose register. However, `BP` is used for the frame pointer and should be callee-save. Under some circumstances, the Go assembler will do this automatically, but not always. At the moment `avo` can produce code that clobbers the `BP` register. Since Go 1.16 this code will also fail a new `go vet` check.

This PR provides a (currently sub-optimal) fix for the issue. It introduces an `EnsureBasePointerCalleeSaved` pass which will check if the base pointer is written to by a function, and if so will artificially ensure that the function has a non-zero frame size. This will trigger the Go assembler to automatically save and restore the BP register.

In addition, we update the `asmdecl` tool to `asmvet`, which includes the `framepointer` vet check.

Updates #156

internal/cmd/asmdecl/main.go

@@ -1,11 +0,0 @@
-// Command asmdecl reports mismatches between assembly files and Go declarations.
-//
-// Standalone version of the static analyzer in go vet.
-package main
-
-import (
-	"golang.org/x/tools/go/analysis/passes/asmdecl"
-	"golang.org/x/tools/go/analysis/singlechecker"
-)
-
-func main() { singlechecker.Main(asmdecl.Analyzer) }

internal/cmd/asmvet/main.go

@@ -0,0 +1,17 @@
+// Command asmvet checks for correctness of Go assembly.
+//
+// Standalone version of the assembly checks in go vet.
+package main
+
+import (
+	"golang.org/x/tools/go/analysis/passes/asmdecl"
+	"golang.org/x/tools/go/analysis/passes/framepointer"
+	"golang.org/x/tools/go/analysis/unitchecker"
+)
+
+func main() {
+	unitchecker.Main(
+		asmdecl.Analyzer,
+		framepointer.Analyzer,
+	)
+}