[DSE] Use optimized access if available for redundant store elimination.

Using the optimized access enables additional optimizations in cases
where the defining access is a non-aliasing store.

Alternatively we could also walk upwards and skip non-aliasing defs
here, but my experiments so far showed that this will noticeably
increase compile-time for little extra gain compared to just using the
optimized access.

Improvements of dse.NumRedundantStores on MultiSource/CINT2006/CPF2006
on X86 with -O3:

     test-suite...-typeset/consumer-typeset.test     1.00                  76.00              7500.0%
     test-suite.../Benchmarks/Bullet/bullet.test     3.00                  12.00              300.0%
     test-suite...006/453.povray/453.povray.test     3.00                   6.00              100.0%
     test-suite...telecomm-gsm/telecomm-gsm.test     1.00                   2.00              100.0%
     test-suite...ediabench/gsm/toast/toast.test     1.00                   2.00              100.0%
     test-suite...marks/7zip/7zip-benchmark.test     1.00                   2.00              100.0%
     test-suite...ications/JM/lencod/lencod.test     7.00                  10.00              42.9%
     test-suite...6/464.h264ref/464.h264ref.test     6.00                   8.00              33.3%
     test-suite...ications/JM/ldecod/ldecod.test     6.00                   7.00              16.7%
     test-suite...006/447.dealII/447.dealII.test    33.00                  33.00               0.0%
     test-suite...6/471.omnetpp/471.omnetpp.test    NaN                     1.00               nan%
     test-suite...006/450.soplex/450.soplex.test    NaN                     2.00               nan%
     test-suite.../CINT2006/403.gcc/403.gcc.test    NaN                     7.00               nan%
     test-suite...lications/ClamAV/clamscan.test    NaN                     1.00               nan%
     test-suite...CI_Purple/SMG2000/smg2000.test    NaN                     3.00               nan%

Follow-up to D111727.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D112315