add loop unrolling optimization pass

[39ali]

this adds a pass that does unrolling for loop using profitability model to make sure we only unroll when it's beneficial

you can run the example : cargo run --example spv-unroll-test examples/spv-unroll-test/shader.spv , shader.spv is based on lib.rs just to have it as a reference

UnrollConfig::default() was chosen for whatever made sense, if anyone has better default values feel free to change it

Currently only 32/64-bit integers work :
f32 / f64: uses OpFOrdLessThan which we don't check for, and float constants need different handling anyway (and floating-point loop bounds aren't statically safe to unroll without exactness guarantees)
u16 / u8: these don't exist as SPIR-V scalar types — they're always widened to u32 in shaders

Negative i32 bounds return None from u64_trip_count due to u32::try_from overflow, so the loop will not unroll.