Re-apply r211399, "Generate native unwind info on Win64" with a fix to ignore SEH pseudo ops in X86 JIT emitter.

--
This patch enables LLVM to emit Win64-native unwind info rather than
DWARF CFI.  It handles all corner cases (I hope), including stack
realignment.

Because the unwind info is not flexible enough to describe stack frames
with a gap of unknown size in the middle, such as the one caused by
stack realignment, I modified register spilling code to place all spills
into the fixed frame slots, so that they can be accessed relative to the
frame pointer.

Patch by Vadim Chugunov!

Reviewed By: rnk

Differential Revision: http://reviews.llvm.org/D4081

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211691 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/MachineFunction.cpp

@@ -578,6 +578,21 @@ int MachineFrameInfo::CreateFixedObject(uint64_t Size, int64_t SPOffset,
   return -++NumFixedObjects;
 }
 
+/// CreateFixedSpillStackObject - Create a spill slot at a fixed location
+/// on the stack.  Returns an index with a negative value.
+int MachineFrameInfo::CreateFixedSpillStackObject(uint64_t Size,
+                                                  int64_t SPOffset) {
+  unsigned StackAlign = getFrameLowering()->getStackAlignment();
+  unsigned Align = MinAlign(SPOffset, StackAlign);
+  Align = clampStackAlignment(!getFrameLowering()->isStackRealignable() ||
+                                  !RealignOption,
+                              Align, getFrameLowering()->getStackAlignment());
+  Objects.insert(Objects.begin(), StackObject(Size, Align, SPOffset,
+                                              /*Immutable*/ true,
+                                              /*isSS*/ true,
+                                              /*Alloca*/ nullptr));
+  return -++NumFixedObjects;
+}
 
 BitVector
 MachineFrameInfo::getPristineRegs(const MachineBasicBlock *MBB) const {