This patch removes spurious includes of `llvm/IR` files, and unnecessary
link components in the LLVM dialect.
The only major dependencies still coming from LLVM are
`llvm::DataLayout`, which is used by `verifyDataLayoutString` and some
`dwarf` symbols in some attributes. Both of them should likely be
removed in the future.
Finally, I also removed one constructor from `LLVM::AssumeOp` that used
[OperandBundleDefT](https://llvm.org/doxygen/classllvm_1_1OperandBundleDefT.html)
without good reason and introduced a header unnecessarily.
This patch replaces uses of StringRef::{starts,ends}with with
StringRef::{starts,ends}_with for consistency with
std::{string,string_view}::{starts,ends}_with in C++20.
I'm planning to deprecate and eventually remove
StringRef::{starts,ends}with.
