[InstCombine] Remove invariant group intrinsincs when comparing against null

We cannot leak any equivalency information by comparing against null
since null never has virtual metadata associated with it (when null is
not a valid dereferenceable pointer).

Instcombine seems to make sure that a null will be on the RHS, so we
don't have to check both operands.

This fixes a missed optimization in llvm-test-suite's MultiSource lambda
benchmark under -fstrict-vtable-pointers.

Reviewed By: Prazek

Differential Revision: https://reviews.llvm.org/D108734