[PATCH] [MachineSink] Use the real post dominator tree

[Jingyue Wu]

Compilation time from llvm-test-suite. Three columns are benchmark name, original compilation time (in seconds), and the slowdown/speedup caused by this change (negative means the new version is faster). I only picked the ones with compilation time longer than one second, because other data points are very noisy. 

I don't see a significant regression in compilation time based on these numbers. 

```
clamscan                                              9.6843             -0.7650%
ldecod                                                4.5576              0.6604%
lencod                                                9.8791              1.5637%
SPASS                                                 9.5813              2.0035%
make_dparser                                          2.0540              1.2723%
kc                                                   10.0371             -0.1296%
lua                                                   2.6484              2.4356%
oggenc                                                3.1777              1.0269%
siod                                                  1.4711              1.3638%
sqlite3                                              12.4904              0.4944%
treecc                                                1.3868             -1.7237%
7zip-benchmark                                       27.6821             -0.2909%
smg2000                                               4.7685              2.4668%
bullet                                               20.0863              0.6708%
espresso                                              3.6563              1.7755%
gs                                                    2.9518             -0.8027%
consumer-jpeg                                         3.4321             -0.4087%
consumer-lame                                         2.8166              2.2606%
consumer-typeset                                      7.7711              0.4109%
office-ispell                                         1.1021              0.0816%
paq8p                                                 1.3805              0.0290%
timberwolfmc                                          6.3654             -1.0692%
mybison                                               1.1789              1.8489%
ControlFlow-dbl                                       1.0986              0.1001%
ControlFlow-flt                                       1.0994             -1.3461%
ControlLoops-dbl                                      1.0301             -1.3881%
ControlLoops-flt                                      1.0259             -0.1170%
CrossingThresholds-dbl                                1.0041             -0.6896%
Equivalencing-flt                                     1.0074             -2.5841%
Expansion-dbl                                         1.0220             -0.4315%
Expansion-flt                                         1.0275              0.0097%
GlobalDataFlow-dbl                                    1.0226             -0.2546%
GlobalDataFlow-flt                                    1.0147             -0.2072%
InductionVariable-dbl                                 1.0076             -0.3878%
LinearDependence-dbl                                  1.0397             -0.0866%
LinearDependence-flt                                  1.0477             -1.8301%
LoopRestructuring-flt                                 1.0237             -0.9422%
Reductions-dbl                                        1.0482              0.1239%
Reductions-flt                                        1.0262              2.6349%
pairlocalalign                                        6.6571             -1.3261%
cjpeg                                                 3.3882              2.9228%
tramp3d-v4                                           15.5355              0.0097%
loop_unroll                                           2.6732              0.4218%
simple_types_constant_folding                         2.1412              0.5031%
simple_types_loop_invariant                           1.7099             -0.3104%
spirit                                                3.0955              0.3676%
```

http://reviews.llvm.org/D5633