CodeGen: Use EmitBranchOnBool when generating For and CXXForRange

A while ago EmitForStmt was changed to explicitly evaluate the
condition expression and create a branch instead of using
EmitBranchOnBool, so that the condition expression could be used for
some cleanup logic. The cleanup stuff has since been reorganized, and
this is no longer necessary.

In EmitCXXForRange, the evaluated condition was never used for
anything else. The logic was presumably modeled on EmitForStmt.

llvm-svn: 193994