[DSE] Support looking through memory phis at end of function.

Update isWriteAtEndOfFunction to look through MemoryPhis. The reason
MemoryPhis were skipped so far was the known AliasAnalysis issue with it
missing loop-carried dependences.

This problem is already addressed in other parts of the code by skipping
MemoryDefs that may be in difference loops. I think the same logic can
be applied here.

This can have a substantial impact on the number of stores removed in
some cases. For MultiSource/SPEC2006/SPEC2017 with -O3:

```
Metric: dse.NumFastStores

Program                                       dse.NumFastStores
                                              base              patch   diff
External/S...CINT2017rate/557.xz_r/557.xz_r     14.00             45.00 221.4%
External/S...te/538.imagick_r/538.imagick_r    439.00           1267.00 188.6%
MultiSourc...e/Applications/SIBsim4/SIBsim4      6.00             15.00 150.0%
MultiSourc...Prolangs-C/simulator/simulator      3.00              7.00 133.3%
MultiSource/Applications/siod/siod               3.00              7.00 133.3%
MultiSourc...arks/FreeBench/distray/distray      6.00              9.00  50.0%
MultiSourc...e/Applications/obsequi/Obsequi     22.00             30.00  36.4%
MultiSource/Benchmarks/Ptrdist/bc/bc            23.00             28.00  21.7%
External/S...NT2017rate/502.gcc_r/502.gcc_r   1258.00           1512.00  20.2%
External/S...te/520.omnetpp_r/520.omnetpp_r    954.00           1143.00  19.8%
External/S...rate/510.parest_r/510.parest_r   5961.00           7122.00  19.5%
External/S...C/CINT2006/445.gobmk/445.gobmk     47.00             56.00  19.1%
External/S...00.perlbench_r/500.perlbench_r    241.00            286.00  18.7%
External/S...NT2006/471.omnetpp/471.omnetpp     36.00             42.00  16.7%
External/S...06/400.perlbench/400.perlbench    183.00            210.00  14.8%
MultiSource/Applications/SPASS/SPASS            72.00             81.00  12.5%
External/S...17rate/541.leela_r/541.leela_r     72.00             80.00  11.1%
External/SPEC/CINT2006/403.gcc/403.gcc         585.00            642.00   9.7%
MultiSourc...e/Applications/sqlite3/sqlite3    120.00            131.00   9.2%
MultiSourc...Applications/hexxagon/hexxagon     11.00             12.00   9.1%
External/S.../CFP2006/453.povray/453.povray    566.00            615.00   8.7%
External/S...rate/511.povray_r/511.povray_r    578.00            627.00   8.5%
External/S...FP2006/482.sphinx3/482.sphinx3     12.00             13.00   8.3%
MultiSource/Applications/oggenc/oggenc         130.00            140.00   7.7%
MultiSourc...e/Applications/ClamAV/clamscan    250.00            268.00   7.2%
MultiSourc.../mediabench/jpeg/jpeg-6a/cjpeg     19.00             20.00   5.3%
MultiSourc...ch/consumer-jpeg/consumer-jpeg     19.00             20.00   5.3%
External/S...te/526.blender_r/526.blender_r   3747.00           3928.00   4.8%
MultiSourc...OE-ProxyApps-C++/miniFE/miniFE    104.00            108.00   3.8%
MultiSourc...ch/consumer-lame/consumer-lame     54.00             56.00   3.7%
MultiSource/Benchmarks/Bullet/bullet          1222.00           1264.00   3.4%
MultiSourc...nchmarks/tramp3d-v4/tramp3d-v4    973.00           1005.00   3.3%
External/S.../CFP2006/447.dealII/447.dealII   2699.00           2780.00   3.0%
External/S...06/483.xalancbmk/483.xalancbmk    788.00            810.00   2.8%
External/S.../CFP2006/450.soplex/450.soplex    180.00            185.00   2.8%
MultiSourc.../DOE-ProxyApps-C++/CLAMR/CLAMR    338.00            345.00   2.1%
MultiSourc...Benchmarks/7zip/7zip-benchmark    685.00            699.00   2.0%
External/S...FP2017rate/544.nab_r/544.nab_r    158.00            160.00   1.3%
MultiSourc...sumer-typeset/consumer-typeset    772.00            781.00   1.2%
External/S...2017rate/525.x264_r/525.x264_r    410.00            414.00   1.0%
External/S...23.xalancbmk_r/523.xalancbmk_r    998.00           1002.00   0.4%
```

Compile-time is almost neutral:

https://llvm-compile-time-tracker.com/compare.php?from=b3125ad3d60531a97eea20009cc9629a87755862&to=84007eee59004f43464eda7f5ba8263ed5158df8&stat=instructions

NewPM-O3: +0.03%
NewPM-ReleaseThinLTO: -0.01%
NewPM-ReleaseLTO-g: +0.03%

Reviewed By: asbirlea

Differential Revision: https://reviews.llvm.org/D132365