Ignore modified attribute list if it yields invalid IR

If modified attribute list is invalid, reverting the change is a
low-cost maintainence solution as compared to examples like
[this](https://github.com/llvm/llvm-project/blob/main/llvm/tools/bugpoint/CrashDebugger.cpp#L368).
This will ensure that the ListReducer maintains the sanctity of any
new attribute dependencies added in the future/already present.

Reviewed By: modocache

Differential Revision: https://reviews.llvm.org/D154348