[LoopUnswitch] Implement first version of partial unswitching.

[LoopUnswitch] Implement first version of partial unswitching.

This patch applies the idea from D93734 to LoopUnswitch.

It adds support for unswitching on conditions that are only
invariant along certain paths through a loop.

In particular, it targets conditions in the loop header that
depend on values loaded from memory. If either path from
the true or false successor through the loop does not modify
memory, perform partial loop unswitching.

That is, duplicate the instructions feeding the condition in the pre-header.
Then unswitch on the duplicated condition. The condition is now known
in the unswitched version for the 'invariant' path through the original loop.

On caveat of this approach is that one of the loops created can be partially
unswitched again. To avoid this behavior, `llvm.loop.unswitch.partial.disable`
metadata is added to the unswitched loops, to avoid subsequent partial
unswitching.

If that's the approach to go, I can move the code handling the metadata kind
into separate functions.

This increases the cases we unswitch quite a bit in SPEC2006/SPEC2000 &
MultiSource. It also allows us to eliminate a dead loop in SPEC2017's omnetpp

```
Tests: 236
Same hash: 170 (filtered out)
Remaining: 66
Metric: loop-unswitch.NumBranches

Program                                        base   patch  diff
 test-suite...000/255.vortex/255.vortex.test     2.00  23.00 1050.0%
 test-suite...T2006/401.bzip2/401.bzip2.test     7.00  55.00 685.7%
 test-suite :: External/Nurbs/nurbs.test         5.00  26.00 420.0%
 test-suite...s-C/unix-smail/unix-smail.test     1.00   3.00 200.0%
 test-suite.../Prolangs-C++/ocean/ocean.test     1.00   3.00 200.0%
 test-suite...tions/lambda-0.1.3/lambda.test     1.00   3.00 200.0%
 test-suite...yApps-C++/PENNANT/PENNANT.test     2.00   5.00 150.0%
 test-suite...marks/Ptrdist/yacr2/yacr2.test     1.00   2.00 100.0%
 test-suite...lications/viterbi/viterbi.test     1.00   2.00 100.0%
 test-suite...plications/d/make_dparser.test    12.00  24.00 100.0%
 test-suite...CFP2006/433.milc/433.milc.test    14.00  27.00 92.9%
 test-suite.../Applications/lemon/lemon.test     7.00  12.00 71.4%
 test-suite...ce/Applications/Burg/burg.test     6.00  10.00 66.7%
 test-suite...T2006/473.astar/473.astar.test    16.00  26.00 62.5%
 test-suite...marks/7zip/7zip-benchmark.test    78.00 121.00 55.1%
```

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D93764