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

reg/types.go

@@ -145,6 +145,7 @@ type Info uint8
 const (
 	None       Info = 0
 	Restricted Info = 1 << iota
+	BasePointer
 )
 
 // Physical is a concrete register.

reg/x86.go

@@ -111,7 +111,7 @@ var (
 
 	// 8-bit
 	SPB  = gp(S8, 4, "SP", Restricted)
-	BPB  = gp(S8, 5, "BP")
+	BPB  = gp(S8, 5, "BP", BasePointer)
 	SIB  = gp(S8, 6, "SI")
 	DIB  = gp(S8, 7, "DI")
 	R8B  = gp(S8, 8, "R8")
@@ -129,7 +129,7 @@ var (
 	DX   = gp(S16, 2, "DX")
 	BX   = gp(S16, 3, "BX")
 	SP   = gp(S16, 4, "SP", Restricted)
-	BP   = gp(S16, 5, "BP")
+	BP   = gp(S16, 5, "BP", BasePointer)
 	SI   = gp(S16, 6, "SI")
 	DI   = gp(S16, 7, "DI")
 	R8W  = gp(S16, 8, "R8")
@@ -147,7 +147,7 @@ var (
 	EDX  = gp(S32, 2, "DX")
 	EBX  = gp(S32, 3, "BX")
 	ESP  = gp(S32, 4, "SP", Restricted)
-	EBP  = gp(S32, 5, "BP")
+	EBP  = gp(S32, 5, "BP", BasePointer)
 	ESI  = gp(S32, 6, "SI")
 	EDI  = gp(S32, 7, "DI")
 	R8L  = gp(S32, 8, "R8")
@@ -165,7 +165,7 @@ var (
 	RDX = gp(S64, 2, "DX")
 	RBX = gp(S64, 3, "BX")
 	RSP = gp(S64, 4, "SP", Restricted)
-	RBP = gp(S64, 5, "BP")
+	RBP = gp(S64, 5, "BP", BasePointer)
 	RSI = gp(S64, 6, "SI")
 	RDI = gp(S64, 7, "DI")
 	R8  = gp(S64, 8, "R8")