[X86][BITREVERSE] Optimized bitreverse builtin for 8 bit scalar integer (PR31810)

[kbelochapka]

Summary

manually optimized code for 8 bit integer bit reverse builtin, instruction count reduced from 14 to 11, slight performance gain

[RKSimon]

The performance implications on PR31810 need to be settled before this can be taken any further

lib/CodeGen/SelectionDAG/LegalizeDAG.cpp

2613

This is general legalization code - we should ensure that ISD::ROTL is legalorcustom before attempting this path, otherwise the additional shift/mask code will definitely be a regression. With that in place you can probably drop the VT.isScalarInteger() requirement.

test/CodeGen/X86/bitreverse.ll

338

Please use the update_llc_test_checks.py script

[kbelochapka]

The performance measurement testing did not show any performance benefit for the proposed 8bit bit reversal intrinsic implementation over the existing one.