[MachineLICM] First steps of sinking GEPs near calls.

Specifically, if there are copy-like instructions in the loop header
they are moved into the loop close to their uses. This reduces the live
intervals of the values and can avoid register spills.

This is working towards a fix for http://llvm.org/PR22230.
Review: http://reviews.llvm.org/D7259

Next steps:
- Find a better cost model (which non-copy instructions should be sunk?)
- Make this dependent on register pressure

llvm-svn: 232262