for each constant pool entry.  Using WriteTypeSymbolic here takes time
proportional to the size of the module, for each constant pool entry.

This speeds up -verbose-asm llc on 252.eon (a random testcase at my disposal)
from 4.4s to 2.137s.  llc takes 2.11s with asm-verbose off, so this is now a
pretty reasonable cost for verbose comments.

llvm-svn: 121691