X86: Do not use ValueMap for PreallocatedIds

[arsenm]

Summary

ValueMap should only be necessary if the IR values can be
replaced. This is only used during codegen, when it's illegal to
change the underlying IR. This allows using the default copy
constructor for X86MachineFunctionInfo.

I'm not happy about targets keeping state here that's only used in one
specific pass, but we don't have a better place to put it right now.

[arsenm]

209e7ef8740838479b79c5b8dfe4d0a820e1e2f0

llvm/lib/Target/X86/X86MachineFunctionInfo.h

class X86MachineFunctionInfo : public MachineFunctionInfo {
   bool HasSwiftAsyncContext = false;
 
   /// True if this function has tile virtual register. This is used to
   /// determine if we should insert tilerelease in frame lowering.
   bool HasVirtualTileReg = false;
 
   Optional<int> SwiftAsyncContextFrameIdx;
 
-  ValueMap<const Value *, size_t> PreallocatedIds;
+  // Preallocated fields are only used during isel.
+  // FIXME: Can we find somewhere else to store these?
+  DenseMap<const Value *, size_t> PreallocatedIds;
   SmallVector<size_t, 0> PreallocatedStackSizes;
   SmallVector<SmallVector<size_t, 4>, 0> PreallocatedArgOffsets;
 
 private:
   /// ForwardedMustTailRegParms - A list of virtual and physical registers
   /// that must be forwarded to every musttail call.
   SmallVector<ForwardedRegister, 1> ForwardedMustTailRegParms;