[llvm] [X86] Remove redundant TEST after shifts when count is non-zero (PR #169069)

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GrumpyPigSkin wrote:

@topperc The MCU results were using alderlake, which is my local machine, after reading what RKSimon sent me it does seems like a lot of work for not much gain for the __builtin_assume case to work.

@RKSimon if I was to remove the changes to SelectionDAG + CodeGenPrepare, but keep the DAG.isKnownNeverZero(Amt) case, since that is guaranteed to not need the test, would you be happy with that? 



https://github.com/llvm/llvm-project/pull/169069