[llvm] [MC] Reduce size of MCLEBFragment (NFC) (PR #75050)

[Fangrui Song via llvm-commits]

MaskRay wrote:

I suggested that we can try `MCEncodedFragmentWithFixups<10, 1>, e.g. <8, 0> <8, 1> <10, 1>.`, perhaps we can just do `<8. 0>`

https://github.com/llvm/llvm-project/pull/75050