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!

llvm-svn: 55969