1. Don't call Value::getName(), which is slow.

2. Lower calls to fabs and friends to FABS nodes etc unless the function has
   internal linkage.  Before we wouldn't lower if it had a definition, which
   is incorrect.  This allows us to compile:

define double @fabs(double %f) {
        %tmp2 = tail call double @fabs( double %f )
        ret double %tmp2
}

into:

_fabs:
        fabs f1, f1
        blr 

llvm-svn: 41805