Potential crash in YARR JIT generated code when building 64 bit
https://bugs.webkit.org/show_bug.cgi?id=110893

Reviewed by Gavin Barraclough.

The ABI doesn't define the behavior for the upper bits of a value that takes less than 64 bits.
Therefore, we zero extend both the count and length registers to assure that these unsigned values
don't have garbage upper bits.

* yarr/YarrJIT.cpp:
(JSC::Yarr::YarrGenerator::generateEnter):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@144083 268f45cc-cd09-0410-ab3c-d52691b4dbfc