[Reassociate] Add initial support for vector instructions.

This patch adds initial support for vector instructions to the reassociation
pass. It enables most parts of the pass to work with vectors but to keep the
size of the patch small, optimization of Xor trees, canonicalization of
negative constants and converting shifts to muls, etc., have been left out.
This will be handled in later patches.

The patch is based on an initial patch by Chad Rosier.

Differential Revision: http://reviews.llvm.org/D7566

llvm-svn: 232190