Extend the vcmp/fcmp LLVM IR instructions to take vectors as arguments

Extend the vcmp/fcmp LLVM IR instructions to take vectors as arguments
and, if so, to return a vector of boolean as a result;

Extend the select LLVM IR instruction to allow you to specify a result
type which is a vector of boolean, in which case the result will be an
element-wise selection instead of choosing one vector or the other; and

Update LangRef.html to describe these changes.

This patch was contributed by Preston Gurd!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@55969 91177308-0d34-0410-b5e6-96231b3b80d8