Revert "[Thumb] Validate branch target for CBZ/CBNZ instructions."

This currently breaks the greendragon clang-stage1-configure-RA/ and
brotli. It is probably just uncovering a pre-existing problem. Reverting
temporarily to get the buildbots green again. A reduced testcase will
follow shortly.

This reverts commit r278659.

llvm-svn: 278711