[X86] SimplifyDemandedBitsForTargetNode - add X86ISD::BZHI handling · llvm/llvm-project#177369 | Good First Issue

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描述

Only the bottom 8 bits of the mask (second operand) are required
Based off the known bits of the mask, we can tweak the src DemandedBits - as well as the incoming DemandedBits mask, if the 8 bit mask is known to be less than bitwidth (getMaxValue.ult(BitWidth)) then bits above getMaxValue are not demanded.

Supporting tests can be added to combine-bzhi.ll

See also #177347 and #177364 for refs

https://www.intel.com/content/www/us/en/docs/intrinsics-guide/index.html#text=bzhi&ig_expand=634,633,5,634

贡献者指南

研究方向: 查看BZHI指令语义（Intel内置指南）。通过将第二个操作数掩码为8位来实现所需位传播。使用已知位分析来减少源和结果的DemandedBits：如果掩码值已知且小于位宽，则getMaxValue以上的位不被需要。在combine bzhi.ll中添加测试。
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