[Codegen] Remove ROCDL index==i32; add indexIsI64 to OptimizeIntArithmetic (#23948)

Integer range analysis now handles narrowing to i32 where safe, making
the --iree-rocm-index-bits option (which lowered all ROCDL indices to
32-bit) obsolete. Remove it so the ROCDL path matches NVVM (which always
has 64-bit indices at the LLVM conversion level).

Add an indexIsI64 option to OptimizeIntArithmeticPass that relaxes the
SAFE_INDEX_UNSIGNED_MAX_VALUE guard on signed-to-unsigned conversions
for index values. On LLVMGPU targets where index is always 64-bit, this
guard is unnecessarily conservative and blocks valid optimizations.
For-loop IV narrowing (NarrowSCFForIvToI32 retains its own range checks
unconditionally.)

Performance impact: on whole models, within the noise floor (as
expected, this killed off a few instructions) but there is a consistent
minor trend on the torch_models CI that gives a 1.01x geometric mean
speedup, so there's not much reason not to do this. Table below.

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Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>