[llvm] [TableGen][MVT] Lower the maximum 16-bit MVT from 16384 to 511. (PR #101401)

[Fangrui Song via llvm-commits]

MaskRay wrote:

The 16-bit MVT patch increased both run time and memory usage greatly: `=time -f '%M %e' -gen-global-isel llvm/lib/Target/RISCV/RISCVGISel.td`
```
1207644 4.80
17415668 16.60
```

Let's try this. If this doesn't make the memory usage down, I think we will need to revert the 16-bit MVT.

https://github.com/llvm/llvm-project/pull/101401