remove the verbose-asm "constant pool double" comments that we were printing

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