[llvm] [NVPTX] Fixup ISel patterns for setcc of i8 extract (PR #151204)

[Alex MacLean via llvm-commits]

AlexMaclean wrote:

> What's the relationship/order between this PR and #150270? They seem to change the same code/tests and seem to have similar effect on the tests.

Sorry for the confusion! After creating https://github.com/llvm/llvm-project/pull/150270 I decided it would make more sense to split out the NVPTX correctness fix from the target-independent DAG-Combiner changes as this is more narrow and should be quicker to review and at lower risk of being reverted due to regressions in other targets. So this is basically a somewhat cleaned up subset of https://github.com/llvm/llvm-project/pull/150270. I'd like to land this first to address the bugs and then rebase the other PR on top of this, at which point it would only contain the combiner improvements. 

https://github.com/llvm/llvm-project/pull/151204