[InstCombine] Canonicalize Bit Testing by Shifting to Bit 0 (#101838)
Implement a new transformation that fold the bit-testing expression
(icmp ne (and (lshr V B) 1) 0) to (icmp ne (and V (shl 1 B)) 0) for
constant V. This rule already existed for non-constant V and constants
other than 1; this restriction to non-constant V has been added in
commit c3b2111d975a39d19f0c5d635e2961a4449c5a71 to fix an infinite loop.
Avoid the infinite loop by allowing constant V only if the shift
instruction is an lshr and the constant is 1. Also fold the negated
variant of the LHS.
This transformation necessitates an adaption of existing tests in
`icmp-and-shift.ll` and `load-cmp.ll`. One test in `icmp-and-shift.ll`,
which previously was a negative test, now gets folded. Rename it to
indicate that it is a positive test.
Alive proof: https://alive2.llvm.org/ce/z/vcJJTx
Relates to issue #86813.
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