[llvm] [InstCombine] Fold redundant select guards for ctpop (PR #195443)

[Sai Sanjay Chikne via llvm-commits]

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+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+;
+; Test folding of redundant early-exit guard around ctpop:
+;   select (icmp ule X, 1), X, ctpop(X)  --> ctpop(X)
+;
+; This fold is valid because ctpop(0)==0 and ctpop(1)==1, so the guard
+; is always redundant. The guard only existed to skip slow software emulation.
+
+; RUN: opt < %s -S -passes=instcombine | FileCheck %s
+
+declare i64 @llvm.ctpop.i64(i64)
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sai18022001 wrote:

@dtcxzyw 
For the intrinsic declarations, `update_test_checks.py` keeps them in the output so I've left them for now. Should I remove them manually?

https://github.com/llvm/llvm-project/pull/195443