Rename loop unrolling and loop vectorizer metadata to have a common prefix.

[Clang part]

These patches rename the loop unrolling and loop vectorizer metadata
such that they have a common 'llvm.loop.' prefix.  Metadata name
changes:

llvm.vectorizer.* => llvm.loop.vectorizer.*
llvm.loopunroll.* => llvm.loop.unroll.*

This was a suggestion from an earlier review
(http://reviews.llvm.org/D4090) which added the loop unrolling
metadata. 

Patch by Mark Heffernan.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@211712 91177308-0d34-0410-b5e6-96231b3b80d8