Loop names used in reporting can grow very large

Summary:
The code for generating a name for loops for various reporting scenarios
created a name by serializing the loop into a string.  This may result in
a very large name for a loop containing many blocks.  Use the getName()
function on the loop instead.

Author: Jamie Schmeiser <schmeise@ca.ibm.com>
Reviewed By: Whitney (Whitney Tsang), aeubanks (Arthur Eubanks)
Differential Revision: https://reviews.llvm.org/D133587