Iasm and obj lowering for advanced switch lowering.

Jump table emission is delayed until offsets are known. X86 local jumps can be
near or far. Sanboxing is applied to indirect jumps from jump table.

BUG=
R=stichnot@chromium.org, jvoung

Review URL: https://codereview.chromium.org/1257283004.
diff --git a/src/IceDefs.h b/src/IceDefs.h
index 4e1fb3a..374b4b3 100644
--- a/src/IceDefs.h
+++ b/src/IceDefs.h
@@ -58,7 +58,9 @@
 class GlobalDeclaration;
 class Inst;
 class InstAssign;
+class InstJumpTable;
 class InstPhi;
+class InstSwitch;
 class InstTarget;
 class LiveRange;
 class Liveness;