TargetInstrInfo: Change duplicate() to work on bundles.

Adds infrastructure to clone whole instruction bundles rather than just
single instructions. This fixes a bug where tail duplication would
unbundle instructions while cloning.

This should unbreak the "Clang Stage 1: cmake, RA, with expensive checks
enabled" build on greendragon. The bot broke with r311139 hitting this
pre-existing bug.

A proper testcase will come next.

llvm-svn: 311511