[llvm] Add support for zero-width integers in MathExtras.h (#87193)

MLIR uses zero-width integers, but also re-uses integer logic from LLVM
to avoid duplication. This creates issues when LLVM logic is used in
MLIR on integers which can be zero-width. In order to avoid
special-casing the bitwidth-related logic in MLIR, this PR adds support
for zero-width integers in LLVM's MathExtras (and consequently APInt).

While most of the logic in theory works the same way out of the box,
because bitshifting right by the entire bitwidth in C++ is undefined
behavior instead of being zero, some special cases had to be added.
Fortunately, it seems like the performance penalty is small. In x86,
this usually yields the addition of a predicated conditional move. I
checked that no branch is inserted in Arm too.

This happens to fix a crash in `arith.extsi` canonicalization in MLIR. I
think a follow-up PR to add tests for i0 in arith would be beneficial.