Add support for undef values in ICE IR.

src/IceTargetLoweringX8632.cpp

Index: src/IceTargetLoweringX8632.cpp
diff --git a/src/IceTargetLoweringX8632.cpp b/src/IceTargetLoweringX8632.cpp
index 46a587fe1f139727438baf153e72f350b3945311..e0a33633423d988bccba9a5e5e7318b80261a15f 100644
--- a/src/IceTargetLoweringX8632.cpp
+++ b/src/IceTargetLoweringX8632.cpp
@@ -2152,6 +2152,12 @@ Operand *TargetX8632::legalize(Operand *From, LegalMask Allowed,
     return From;
   }
   if (llvm::isa<Constant>(From)) {
+    if (llvm::isa<ConstantUndef>(From)) {
+      // Lower undefs to zero. Another option is to lower undefs to an
+      // uninitialized register; however, using an uninitialized register
+      // results in less predictable code.
+      From = Ctx->getConstantZero(From->getType());
+    }
     if (!(Allowed & Legal_Imm)) {
       Variable *Reg = makeReg(From->getType(), RegNum);
       _mov(Reg, From);