[ShrinkWrap] Allow shrinkwrapping past memory accesses to jump tables

This patch adds a check for whether the memory operand is known to be
a jump table and, if so, allows shrinkwrapping to continue. In the
case that we are looking at a jump table, I believe it is safe to
assume that the access will not be to the stack (but please correct me
if I am wrong here).

In the test attached, this is helpful in that we are able to generate
only one instruction for each non-default case in the original switch
statement.

Differential Revision: https://reviews.llvm.org/D149886