[ScheduleDAGInstrs] Compute topological ordering on demand.

In most cases, the topological ordering does not get changed in
ScheduleDAGInstrs. We can compute the ordering on demand, similar to
D60125.

This drastically cuts down the number of times we need to compute the
topological ordering, e.g. for SPEC2006, SPEC2k and MultiSource, we get
the following stats for -O3 -flto on X86 (showing the top reductions,
with small absolute values filtered). The smallest reduction is -50%.

Slightly positive impact on compile-time (-0.1 % geomean speedup for
test-suite + SPEC & co, with -O1 on X86)

Tests: 243
Metric: pre-RA-sched.NumTopoInits

Program                                        base       patch  diff
 test-suite...ngs-C/fixoutput/fixoutput.test   115.00      3.00   -97.4%
 test-suite...ks/Prolangs-C/cdecl/cdecl.test   957.00     26.00   -97.3%
 test-suite...math/automotive-basicmath.test   107.00      3.00   -97.2%
 test-suite...rolangs-C++/deriv2/deriv2.test   144.00      6.00   -95.8%
 test-suite...lowfish/security-blowfish.test   410.00     18.00   -95.6%
 test-suite...frame_layout/frame_layout.test   441.00     23.00   -94.8%
 test-suite...rolangs-C++/employ/employ.test   159.00     11.00   -93.1%
 test-suite...s/Ptrdist/anagram/anagram.test   157.00     11.00   -93.0%
 test-suite...s-C/unix-smail/unix-smail.test   829.00     59.00   -92.9%
 test-suite...chmarks/Olden/power/power.test   154.00     11.00   -92.9%
 test-suite...T95/147.vortex/147.vortex.test   19876.00  1434.00  -92.8%
 test-suite...000/255.vortex/255.vortex.test   19881.00  1435.00  -92.8%
 test-suite...ce/Applications/Burg/burg.test   2203.00   168.00   -92.4%
 test-suite...urce/Applications/hbd/hbd.test   1067.00    85.00   -92.0%
 test-suite...ternal/HMMER/hmmcalibrate.test   3145.00   251.00   -92.0%
 test-suite.../Applications/spiff/spiff.test   1037.00    84.00   -91.9%
 test-suite...SPEC/CINT95/130.li/130.li.test   5913.00   487.00   -91.8%
 test-suite.../CINT95/134.perl/134.perl.test   12532.00  1041.00  -91.7%
 test-suite...ce/Benchmarks/Olden/bh/bh.test   220.00     19.00   -91.4%
 test-suite :: External/Nurbs/nurbs.test       2304.00   206.00   -91.1%
 test-suite...arks/VersaBench/dbms/dbms.test   773.00     75.00   -90.3%
 test-suite...ce/Applications/siod/siod.test   9043.00   878.00   -90.3%
 test-suite...pplications/treecc/treecc.test   4510.00   438.00   -90.3%
 test-suite...T2006/456.hmmer/456.hmmer.test   7093.00   697.00   -90.2%
 test-suite...s-C/Pathfinder/PathFinder.test   882.00     87.00   -90.1%
 test-suite.../CINT2000/176.gcc/176.gcc.test   64978.00  6721.00  -89.7%
 test-suite...cations/hexxagon/hexxagon.test   657.00     69.00   -89.5%
 test-suite...fice-ispell/office-ispell.test   2712.00   285.00   -89.5%
 test-suite.../CINT2006/403.gcc/403.gcc.test   139613.00 14992.00 -89.3%
 test-suite...lications/ClamAV/clamscan.test   25880.00  2785.00  -89.2%

Reviewers: MatzeB, atrick, efriedma, niravd

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D60839

llvm-svn: 361253