[DSE] Do not scan users of memory terminators for further reads.

isMemTerminator checks if the current def is a memory terminator that
terminates the memory pointed to by DefLoc. We do not have to add any of
their users to the worklist, because the follow-on users cannot read the
memory in question.

This leads to more stores eliminated in the presence of lifetime calls.
Previously we added the users of those intrinsics to the worklist,
limiting elimination.

In terms of removed stores, this gives a nice boost on some benchmarks
(MultiSource/SPEC2000/SPEC2006 on X86 with -flto -O3):

Same hash: 205 (filtered out)
Remaining: 32
Metric: dse.NumFastStores

Program                                          base   patch   diff
 test-suite...000/197.parser/197.parser.test     4.00    8.00  100.0%
 test-suite...rolangs-C++/family/family.test     4.00    7.00  75.0%
 test-suite...marks/7zip/7zip-benchmark.test   1722.00 2189.00 27.1%
 test-suite...CFP2000/177.mesa/177.mesa.test    30.00   38.00  26.7%
 test-suite :: External/Nurbs/nurbs.test        44.00   49.00  11.4%
 test-suite...lications/sqlite3/sqlite3.test   115.00  128.00  11.3%
 test-suite...006/447.dealII/447.dealII.test   2715.00 3013.00 11.0%
 test-suite...ProxyApps-C++/CLAMR/CLAMR.test   237.00  261.00  10.1%
 test-suite...tions/lambda-0.1.3/lambda.test    40.00   44.00  10.0%
 test-suite...3.xalancbmk/483.xalancbmk.test   1366.00 1475.00  8.0%
 test-suite...abench/jpeg/jpeg-6a/cjpeg.test    13.00   14.00   7.7%
 test-suite...oxyApps-C++/miniFE/miniFE.test    43.00   46.00   7.0%
 test-suite...lications/ClamAV/clamscan.test   230.00  246.00   7.0%
 test-suite...006/450.soplex/450.soplex.test   284.00  299.00   5.3%
 test-suite...nsumer-jpeg/consumer-jpeg.test    21.00   22.00   4.8%