add a fastpath to ConstantExpr::getBitCast to handle the case when an obviously

unneeded bitcast is requested.  This is common for frontends who just unconditionally
cast even if the target is often the right type already.  THis prevents going into
getFoldedCast which switches on the opcode and does a bunch of other stuff before
doing the same opzn.

llvm-svn: 67435

llvm/lib/VMCore/Constants.cpp

@@ -1996,6 +1996,11 @@ Constant *ConstantExpr::getBitCast(Constant *C, const Type *DstTy) {
   unsigned DstBitSize = DstTy->getPrimitiveSizeInBits();
 #endif
   assert(SrcBitSize == DstBitSize && "BitCast requires types of same width");
+  
+  // It is common to ask for a bitcast of a value to its own type, handle this
+  // speedily.
+  if (C->getType() == DstTy) return C;
+  
   return getFoldedCast(Instruction::BitCast, C, DstTy);
 }