Convert code to compile with vc7.1.

Patch contributed by Paolo Invernizzi. Thanks Paolo!

llvm-svn: 16368
diff --git a/llvm/lib/Target/X86/X86ISelSimple.cpp b/llvm/lib/Target/X86/X86ISelSimple.cpp
index e9dbc4b..c3a63fe 100644
--- a/llvm/lib/Target/X86/X86ISelSimple.cpp
+++ b/llvm/lib/Target/X86/X86ISelSimple.cpp
@@ -643,8 +643,8 @@
 
     // Loop over all of the PHI nodes in the LLVM basic block...
     MachineBasicBlock::iterator PHIInsertPoint = MBB.begin();
-    for (BasicBlock::const_iterator I = BB->begin();
-         PHINode *PN = const_cast<PHINode*>(dyn_cast<PHINode>(I)); ++I) {
+    for (BasicBlock::const_iterator I = BB->begin(); isa<PHINode>(I); ++I) {
+      PHINode *PN = const_cast<PHINode*>(dyn_cast<PHINode>(I));
 
       // Create a new machine instr PHI node, and insert it.
       unsigned PHIReg = getReg(*PN);