[NFC] Rename isPointerOffset to getPointerOffsetFrom and move to Value.h

Linking LLVMCore failed when building D148536 with shared libs enabled:
https://lab.llvm.org/buildbot/#/builders/121/builds/29766

Make isPointerOffset a Value method and rename it to getPointerOffsetFrom.

Reviewed By: jmorse

Differential Revision: https://reviews.llvm.org/D148698