[InstCombine] Fix worklist management when removing guard intrinsic

When multiple guard intrinsics are merged into one, currently the
result of eraseInstFromFunction() is returned -- however, this
should only be done if the current instruction is being removed.
In this case we're removing a different instruction and should
instead report that the current one has been modified by returning it.

For this test case, this reduces the number of instcombine iterations
from 5 to 2 (the minimum possible).

Differential Revision: https://reviews.llvm.org/D72558