[RS4GC] Add a test to demonstrate duplication of base generation. NFC

This new test demonstrates a case where a base ptr is generated
twice for the same value: the first one is generated while
the gc.get.pointer.base() is inlined, the second is generated
for the statepoint. This happens because the methods
inlineGetBaseAndOffset() and insertParsePoints() do not share
their defining value cache used by the findBasePointer() method.

Reviewed By: reames
Differential Revision: https://reviews.llvm.org/D103238