reg: support for register casting

Adds methods for referencing sub- or super-registers. For example, for
general purpose registers you can now reference As8(), As16(), ... and
for vector AsX(), AsY(), AsZ().

Closes #1

tests/cast/asm.go

@@ -0,0 +1,24 @@
+// +build ignore
+
+package main
+
+import (
+	. "github.com/mmcloughlin/avo/build"
+)
+
+func main() {
+	TEXT("Split", "func(x uint64) (q uint64, l uint32, w uint16, b uint8)")
+	Doc(
+		"Split returns the low 64, 32, 16 and 8 bits of x.",
+		"Tests the As() methods of virtual general-purpose registers.",
+	)
+	x := GP64v()
+	Load(Param("x"), x)
+	Store(x, Return("q"))
+	Store(x.As32(), Return("l"))
+	Store(x.As16(), Return("w"))
+	Store(x.As8(), Return("b"))
+	RET()
+
+	Generate()
+}

tests/cast/cast.s

@@ -0,0 +1,12 @@
+// Code generated by command: go run asm.go -out cast.s -stubs stub.go. DO NOT EDIT.
+
+#include "textflag.h"
+
+// func Split(x uint64) (q uint64, l uint32, w uint16, b uint8)
+TEXT ·Split(SB), 0, $0-23
+	MOVQ	x(FP), AX
+	MOVQ	AX, q+8(FP)
+	MOVL	AX, l+16(FP)
+	MOVW	AX, w+20(FP)
+	MOVB	AL, b+22(FP)
+	RET

tests/cast/cast_test.go

@@ -0,0 +1,17 @@
+package cast
+
+import (
+	"testing"
+	"testing/quick"
+)
+
+//go:generate go run asm.go -out cast.s -stubs stub.go
+
+func TestSplit(t *testing.T) {
+	expect := func(x uint64) (uint64, uint32, uint16, uint8) {
+		return x, uint32(x), uint16(x), uint8(x)
+	}
+	if err := quick.CheckEqual(Split, expect, nil); err != nil {
+		t.Fatal(err)
+	}
+}

tests/cast/stub.go

@@ -0,0 +1,7 @@
+// Code generated by command: go run asm.go -out cast.s -stubs stub.go. DO NOT EDIT.
+
+package cast
+
+// Split returns the low 64, 32, 16 and 8 bits of x.
+// Tests the As() methods of virtual general-purpose registers.
+func Split(x uint64) (q uint64, l uint32, w uint16, b uint8)