https://bugs.webkit.org/show_bug.cgi?id=122538

Reviewed by Oliver Hunt.
        
This gives the FTL the ability to OSR exit using the llvm.webkit.stackmap intrinsic.
        
- The FTL compiles all OSR exit calls as calls to llvm.webkit.stackmap with a unique
  ID, passing a requested size that is big enough for own jump replacement.
        
- After LLVM compilation, we parse the new LLVM stackmap section.
        
- For all llvm.webkit.stackmaps that we used for OSR exits, we do a jumpReplacement,
  which targets exit thunks that we generate.
        
- If an exit thunk fires, it causes JSC to compile an exit off-ramp that uses a
  combination of the JSC-internal OSR exit accounting (FTL::ExitValue and friends) and
  LLVM stackmap's accounting of where data actually ended up (register, indirect,
  constant) to reconstruct bytecode state.
        
This still has shortcomings; for example it cannot handle XMM or YMM registers. Handling
Y...