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-cth bit.