[SamplePGO] Fix ICE that callee samples returns null while finding import functions

We found that in a special condition, the input callee `Samples` is null for `findExternalInlineCandidate`, which

[SamplePGO] Fix ICE that callee samples returns null while finding import functions

We found that in a special condition, the input callee `Samples` is null for `findExternalInlineCandidate`, which caused an ICE.

In some rare cases, call instruction could be changed after being pushed into inline candidate queue, this is because earlier inlining may expose constant propagation which can change indirect call to direct call. When this happens, we may fail to find matching function samples for the candidate later(for example if the profile is stale), even if a match was found when the candidate was enqueued.

See this reduced program:

file1.c:
```
int bar(int x);

int(*foo())() {
return bar;
};

void func()
{
int (*fptr)(int);
fptr = foo();
a += (*fptr)(10);
}
```
file2.c:
```
int bar(int x) { return x + 1;}
```

The two CALL: `foo` and `(*ptr)` are pushed into the queue at the beginning, say `foo` is hotter and popped first for inlining. During the inlining of `foo`, it performs the constant propagation for the function pointer `bar` and then changed `(*ptr)` to a direct call `bar(..)`. Note that at this time, `(*ptr)/bar` is still in the queue, later while it's popped out for inlining, it use the a different target name(bar) to look for the callee samples. At the same time, if the profile is stale and the new function is different from the old function in the profile, then this led the return of the null callee sample.

Reviewed By: hoy, wenlei

Differential Revision: https://reviews.llvm.org/D154637

show more ...