[FastISel][ARM] Do not emit stores for undef arguments.

This is a followup patch for r214366, which added the same behavior to the
AArch64 and X86 FastISel code. This fix reproduces the already existing
behavior of SelectionDAG in FastISel.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214531 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Target/ARM/ARMFastISel.cpp

@@ -1941,6 +1941,7 @@ bool ARMFastISel::ProcessCallArgs(SmallVectorImpl<Value*> &Args,
   // Process the args.
   for (unsigned i = 0, e = ArgLocs.size(); i != e; ++i) {
     CCValAssign &VA = ArgLocs[i];
+    const Value *ArgVal = Args[VA.getValNo()];
     unsigned Arg = ArgRegs[VA.getValNo()];
     MVT ArgVT = ArgVTs[VA.getValNo()];
 
@@ -2001,6 +2002,11 @@ bool ARMFastISel::ProcessCallArgs(SmallVectorImpl<Value*> &Args,
     } else {
       assert(VA.isMemLoc());
       // Need to store on the stack.
+
+      // Don't emit stores for undef values.
+      if (isa<UndefValue>(ArgVal))
+        continue;
+
       Address Addr;
       Addr.BaseType = Address::RegBase;
       Addr.Base.Reg = ARM::SP;