[mlir] Take ValueRange instead of ArrayRef<Value> in StructuredIndexed

This was likely overlooked when ValueRange was first introduced. There is no
reason why StructuredIndexed needs specifically an ArrayRef so use ValueRange
for better type compatibility with the rest of the APIs.

Reviewed By: nicolasvasilache, mehdi_amini

Differential Revision: https://reviews.llvm.org/D87127