Index types integers of platform-specific bit width. They are used to index
memrefs and as loop induction variables, however they could not be obtained
from an integer until now, making it virtually impossible to express indirect
accesses (given that memrefs of indices are not allowed) or data-dependent
loops. Introduce `std.index_cast` to transform indices into integers and vice
versa. The semantics of this cast is to sign-extend when casting to a wider
integer, and to truncate when casting to a narrower integer. It belongs to
StandardOps because both types it operates on are standard types, and because
its results are likely to be used in std.load and std.store.
Introduce llvm.sext, llvm.zext and llvm.trunc operations to the LLVM dialect.
Provide the conversion of `std.index_cast` to llvm.sext or llvm.trunc,
depending on the actual bitwidth of `index` known during the conversion.
PiperOrigin-RevId: 253624100
Conversions from dialect A to dialect B depend on both A and B. Therefore, it
is reasonable for them to live in a separate library that depends on both
DialectA and DialectB library, and does not forces dependees of DialectA or
DialectB to also link in the conversion. Create the directory layout for the
conversions and move the Standard to LLVM dialect conversion as the first
example.
PiperOrigin-RevId: 253312252
