[InlineCost] Addressing a very strict assert check in CostAnnotationWriter::emitInstructionAnnot

The assert checks that every instruction must be annotated by this point while it is not
necessary. If the inlining process was interrupted because the threshold was reached, the rest
of the instructions would not be annotated which triggers the assert.
The added test shows the situation in which it can happen.

Reviewed-By: mtrofin
Diff: https://reviews.llvm.org/D79107