Teach IRCE to look at branch weights when recognizing range checks

Splitting a loop to make range checks redundant is profitable only if
the range check "never" fails. Make this fact a part of recognizing a
range check -- a branch is a range check only if it is expected to
pass (via branch_weights metadata).

Differential Revision: http://reviews.llvm.org/D7192

llvm-svn: 227249