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()
+}