[SlotIndexes] Add print-slotindexes to disable printing slotindexes

Summary:
When we print the IR with --print-after/before-*,
SlotIndexes will be printed whenever available (We haven't freed it).

This introduces some noises when we try to compare the IR
among different optimizations.

eg:
-print-before=machine-cp will print SlotIndexes for 1st machine-cp
pass, but NOT for 2nd machine-cp;
-print-after=machine-cp will NOT print SlotIndexes for both
machine-cp passes.
So SlotIndexes in 1st pass introduce noises when differing these IRs.

This patch introduces an option to hide indexes.

Reviewers: stoklund, thegameg, qcolombet

Reviewed By: thegameg

Subscribers: hiraditya, arphaman, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66500

llvm-svn: 369650