[mlir] Bump SmallVector sizes along hot paths (#188827)
This is based on empirical data from compiling 9 medium to large
language and diffusion models with IREE. e2e, this improves compilation
times by 0.33% in terms of `instructions:u` (same metric is used by the
[CTMark for
Clang](https://www.npopov.com/2024/01/01/This-year-in-LLVM-2023.html#compile-time-improvements)).
I explored using other constants and these are the ones that performed
best while keeping the sizes relatively small.
