[MustExecute] checkForAllContext(): use pre-increment

[MustExecute] checkForAllContext(): use pre-increment

Summary:
You'd think there is no difference, but this halves (yikes!) compiler memory usage
on `test-suite/MultiSource/Applications/SPASS/top.c` test,
because `MustBeExecutedIterator operator++()` is, well, post-increment,
it must create a duplicate of existing `MustBeExecutedIterator`,
which involves duplicating `VisitedSetTy Visited` which is `DenseSet`..

Old
```
   0.3573 ( 42.9%)   0.0264 ( 33.7%)   0.3837 ( 42.1%)   0.3837 ( 42.1%)  Deduce and propagate attributes (CGSCC pass)
   0.1011 ( 12.1%)   0.0199 ( 25.4%)   0.1210 ( 13.3%)   0.1210 ( 13.3%)  Deduce and propagate attributes
```
```
total runtime: 20.04s.
bytes allocated in total (ignoring deallocations): 1.09GB (54.63MB/s)
calls to allocation functions: 1142410 (57020/s)
temporary memory allocations: 500538 (24983/s)
peak heap memory consumption: 26.68MB
peak RSS (including heaptrack overhead): 944.85MB
total memory leaked: 8.85MB
```
New:
```
   0.3309 ( 39.8%)   0.0164 ( 33.3%)   0.3473 ( 39.5%)   0.3473 ( 39.5%)  Deduce and propagate attributes (CGSCC pass)
   0.1152 ( 13.9%)   0.0076 ( 15.5%)   0.1229 ( 14.0%)   0.1229 ( 14.0%)  Deduce and propagate attributes
```
```
total runtime: 19.49s.
bytes allocated in total (ignoring deallocations): 575.07MB (29.51MB/s)
calls to allocation functions: 909059 (46651/s)
temporary memory allocations: 276923 (14211/s)
peak heap memory consumption: 26.68MB
peak RSS (including heaptrack overhead): 942.90MB
total memory leaked: 8.85MB
```
Diff:
```
total runtime: -0.55s.
bytes allocated in total (ignoring deallocations): -519.41MB (946.11MB/s)
calls to allocation functions: -233351 (425047/s)
temporary memory allocations: -223615 (407313/s)
peak heap memory consumption: 0B
peak RSS (including heaptrack overhead): 0B
total memory leaked: 0B
```

Reviewers: jdoerfert

Reviewed By: jdoerfert

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D78225