[llvm] [GlobalISel][TableGen] MIR Pattern Variadics (PR #100563)

[Thorsten Schütt via llvm-commits]

tschuett wrote:

```G_MUL $x, GIVariadic<>:$lhsAndRhs```should give an error. G_MUL is not variadic. It is a misuse of the feature.

https://github.com/llvm/llvm-project/pull/100563