[OpenMP] target nested `use_device_ptr() if()` and is_device_ptr trigger asserts

Clang now asserts for the below case:
```
void clang::CodeGen::CGOpenMPRuntime::createOffloadEntriesAndInfoMetadata(): Assertion `std::get<0>(E) && "All ordered entries must exist!"' failed.
```

The reason why Clang hit the assert is because in
`emitTargetDataCalls`, both `BeginThenGen` and `BeginElseGen` call
`registerTargetRegionEntryInfo` and try to register the Entry in
OffloadEntriesTargetRegion with same key. If changing the expression in
if clause to any constant expression, then the assert disappear. (https://godbolt.org/z/TW7haj)

The assert itself is to avoid
user from accessing elements out of bound inside `OrderedEntries` in
`createOffloadEntriesAndInfoMetadata`.

In this patch, I add a check in `registerTargetRegionEntryInfo` to avoid
register the target region more than once.

A test case that triggers assert: https://godbolt.org/z/4cnGW8

Reviewed By: ABataev

Differential Revision: https://reviews.llvm.org/D90704