[FuzzMutate] RandomIRBuilder has more source and sink type now.

Source and Sink are required when generating a new instruction.
(Term defined by previous author, in LLVM terms it's probably Use and User.)
Previously, only instructions in the same block is considered when taking source and sink.

In this patch, more source and sink types are considered.
For source, we have SrcFromInstInCurBlock, FunctionArgument, InstInDominator, SrcFromGlobalVariable, and NewConstOrStack.
For sink, we have SinkToInstInCurBlock, PointersInDominator, InstInDominatee, NewStore, and SinkToGlobalVariable.

A unit test to make sure source always dominates an instruction, and the instruction always dominates the sink is included.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D139907