Create a separate file for JS-specific unit tests.

Reviewers: djasper

Reviewed By: djasper

CC: cfe-commits, klimek

Differential Revision: http://llvm-reviews.chandlerc.com/D2307
[SystemZ] Fix choice of known-zero mask in insertion optimization

The backend converts 64-bit ORs into subreg moves if the upper 32 bits
of one operand and the low 32 bits of the other are known to be zero.
It then tries to peel away redundant ANDs from the upper 32 bits.

Since AND masks are canonicalized to exclude known-zero bits,
the test ORs the mask and the known-zero bits together before
checking for redundancy.  The problem was that it was using the
wrong node when checking for known-zero bits, so could drop ANDs
that were still needed.
Addrspacecasts are no-ops on ARM.

Testcase added.