[MLIR][VectorToLLVM] Handle scalable dim in createVectorLengthValue() (#93361)

LLVM's Vector Predication Intrinsics require an explicit vector length
parameter:
https://llvm.org/docs/LangRef.html#vector-predication-intrinsics.

For a scalable vector type, this should be caculated as VectorScaleOp
multiplied by base vector length, e.g.: for <[4]xf32> we should return:
vscale * 4.