[MemoryBuiltins] Accept any value in getInitialValueOfAllocation() (NFC)

Drop the requirement that getInitialValueOfAllocation() must be
passed an allocator function, shifting the responsibility for
checking that into the function (which it does anyway). The
motivation is to avoid some calls to isAllocationFn(), which has
somewhat ill-defined semantics (given the number of
allocator-related attributes we have floating around...)

(For this function, all we eventually need is an allockind of
zeroed or uninitialized.)

Differential Revision: https://reviews.llvm.org/D127274