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/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