[X86] Update an outdated comment about SjLj. NFC.

[mstorsjo]

Summary

The SjLj intrinsics in the X86 backend are intended for use with SjLj exception handling as well, since SVN r271244.

[compnerd]

Id like to keep the bit about "please don't use these" ... we do support zero cost exceptions nearly everywhere now.

[mstorsjo]

Updated the comment as requested.

lib/Target/X86/X86ISelLowering.cpp

for (auto VT : { MVT::i8, MVT::i16, MVT::i32, MVT::i64 }) {
     setOperationAction(ISD::SETCC,  VT, Custom);
   }
 
   // Custom action for SELECT MMX and expand action for SELECT_CC MMX
   setOperationAction(ISD::SELECT, MVT::x86mmx, Custom);
   setOperationAction(ISD::SELECT_CC, MVT::x86mmx, Expand);
 
   setOperationAction(ISD::EH_RETURN       , MVT::Other, Custom);
-  // NOTE: EH_SJLJ_SETJMP/_LONGJMP supported here is NOT intended to support
-  // SjLj exception handling but a light-weight setjmp/longjmp replacement to
-  // support continuation, user-level threading, and etc.. As a result, no
-  // other SjLj exception interfaces are implemented and please don't build
-  // your own exception handling based on them.
-  // LLVM/Clang supports zero-cost DWARF exception handling.
+  // NOTE: EH_SJLJ_SETJMP/_LONGJMP are not recommended, since
+  // LLVM/Clang supports zero-cost DWARF and SEH exception handling.
   setOperationAction(ISD::EH_SJLJ_SETJMP, MVT::i32, Custom);
   setOperationAction(ISD::EH_SJLJ_LONGJMP, MVT::Other, Custom);
   setOperationAction(ISD::EH_SJLJ_SETUP_DISPATCH, MVT::Other, Custom);
   if (TM.Options.ExceptionModel == ExceptionHandling::SjLj)
     setLibcallName(RTLIB::UNWIND_RESUME, "_Unwind_SjLj_Resume");
 
   // Darwin ABI issue.
   for (auto VT : { MVT::i32, MVT::i64 }) {