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fdb6466319
This implements the following optimization: ``` (tbz (shl x, c), b) -> (tbz x, b-c) ``` Which appears in `getTestBitOperand` in AArch64ISelLowering.cpp. If we test bit `b` of `shl x, c`, we can fold away the `shl` by looking `c` bits to the right of `b` in `x` when this fits in the type. So, we can just test the `b-c`th bit. Differential Revision: https://reviews.llvm.org/D73924