MCInst: decrease inline element count to 6. NFC

MCInst is primarily used in local variables and MCRelaxableFragment
(mostly JMP/JCC for x86). Reducing the inline element count can make
MCRelaxableFragment smaller, potentially leading to a lower peak RSS.

When compiling sqlite3.c, x86-64 has the largest maximum numOperands.

aarch64: 5; ppc64: 6; riscv64: 3; s390x: 6; x86-64: 8

Here is the frequency table for x86-64:

max getNumOperands: 8
0: 676
1: 37892
2: 84046
3: 26767
4: 1640
5: 1222
6: 80794
7: 768
8: 22

Pull Request: https://github.com/llvm/llvm-project/pull/94913