[instcombine] Exploit UB implied by nofree attributes

This patch simply implements the documented UB of the current nofree attributes as specified. It doesn't try to be fancy about inference (yet), it just implements the cases already specified and inferred.

Note: When this lands, it may expose miscompiles. If so, please revert and provide a test case. It's likely the bug is in the existing inference code and without a relatively complete test case, it will be hard to debug.

Differential Revision: https://reviews.llvm.org/D96349