The FTL allocated spill slots for BinaryOps is sometimes inaccurate.

The FTL allocated spill slots for BinaryOps is sometimes inaccurate.
https://bugs.webkit.org/show_bug.cgi?id=152918

Reviewed by Filip Pizlo and Saam Barati.

* ftl/FTLCompile.cpp:
- Updated a comment.
* ftl/FTLLowerDFGToLLVM.cpp:
(JSC::FTL::DFG::LowerDFGToLLVM::lower):
- The code to compute maxNumberOfCatchSpills was unnecessarily allocating an
  extra slot for BinaryOps that don't have Untyped operands, and failing to
  allocate that extra slot for some binary ops.  This is now fixed.

* tests/stress/ftl-shr-exception.js:
* tests/stress/ftl-xor-exception.js:
- Un-skipped these tests.  They now pass with this patch.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@194820 268f45cc-cd09-0410-ab3c-d52691b4dbfc