Remove 'set frame pointer' unwind code from Windows x64 unwind.

This commit removes the "set frame pointer" unwind code and frame
pointer information from Windows x64 unwind information.

In Windows x64 unwind information, a "frame pointer" is actually the
*base address* of the static part of the local frame and would be at some
negative offset to RSP upon establishing the frame pointer.

Currently Cranelift uses a "traditional" notion of a frame pointer, one
that is the highest address in the local frame (i.e. pointing at the
previous frame pointer on the stack).

Windows x64 unwind doesn't describe such frame pointers and only needs
one described if the frame contains a dynamic stack allocation.

Fixes #1967.

cranelift/codegen/src/isa/unwind/winx64.rs

@@ -57,10 +57,6 @@ pub(crate) enum UnwindCode {
         offset: u8,
         size: u32,
     },
-    SetFramePointer {
-        offset: u8,
-        sp_offset: u8,
-    },
 }
 
 impl UnwindCode {
@@ -69,7 +65,6 @@ impl UnwindCode {
             PushNonvolatileRegister = 0,
             LargeStackAlloc = 1,
             SmallStackAlloc = 2,
-            SetFramePointer = 3,
             SaveXmm128 = 8,
             SaveXmm128Far = 9,
         }
@@ -113,10 +108,6 @@ impl UnwindCode {
                     writer.write_u32::<LittleEndian>(*size);
                 }
             }
-            Self::SetFramePointer { offset, sp_offset } => {
-                writer.write_u8(*offset);
-                writer.write_u8((*sp_offset << 4) | (UnwindOperation::SetFramePointer as u8));
-            }
         };
     }