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[InstCombine] Fold Minimum over Trailing/Leading Bits Counts (#90000)
The new transformation folds `umin(cttz(x), c)` to `cttz(x | (1 << c))` and `umin(ctlz(x), c)` to `ctlz(x | ((1 << (bitwidth - 1)) >> c))`. The transformation is only implemented for constant `c` to not increase the number of instructions. The idea of the transformation is to set the c-th lowest (for `cttz`) or highest (for `ctlz`) bit in the operand. In this way, the `cttz` or `ctlz` instruction always returns at most `c`. Alive2 proofs: https://alive2.llvm.org/ce/z/y8Hdb8
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