[ShrinkWrap] Use underlying object to rule out stack access.

Allow shrink-wrapping past memory accesses that only access globals or
function arguments. This patch uses getUnderlyingObject to try to
identify the accessed object by a given memory operand. If it is a
global or an argument, it does not access the stack of the current
function and should not block shrink wrapping.

Note that the caller's stack may get accessed when passing an argument
via the stack, but not the stack of the current function.

This addresses part of the TODO from D63152.

Reviewed By: thegameg

Differential Revision: https://reviews.llvm.org/D149668