[MLIR][NFC] Fix for VS2017 bug.

Apparently there is a bug in VS2017 concerning compile-time constant
expressions.
https://developercommunityapi.westus.cloudapp.azure.com/content/problem/110435/not-compile-time-constant-expression.html

This change works with any C++11 compiler (including VS2017) and is a little bit
easier to read.

Reviewers: stella.stamenova, nicolasvasilache

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D75584