Support SIZEOF(reg) to distinguish 8- and 16-bit registers (#1758)

test/asm/sizeof-reg.asm

@@ -0,0 +1,30 @@
+assert sizeof(a) == 1
+assert sizeof(b) == 1
+assert sizeof(c) == 1
+assert sizeof(d) == 1
+assert sizeof(e) == 1
+assert sizeof(h) == 1
+assert sizeof(l) == 1
+
+assert sizeof([bc]) == 1
+assert sizeof([de]) == 1
+assert sizeof([hl]) == 1
+
+assert sizeof([hli]) == 1
+assert sizeof([hl+]) == 1
+assert sizeof([hld]) == 1
+assert sizeof([hl-]) == 1
+
+assert sizeof(af) == 2
+assert sizeof(bc) == 2
+assert sizeof(de) == 2
+assert sizeof(hl) == 2
+assert sizeof(sp) == 2
+
+assert sizeof(high(af)) == 1
+assert sizeof(high(bc)) == 1
+assert sizeof(low(bc)) == 1
+assert sizeof(high(de)) == 1
+assert sizeof(low(de)) == 1
+assert sizeof(high(hl)) == 1
+assert sizeof(low(hl)) == 1