[MLIR][NFC] Fix a checked after used case in ReshapeOpsUtils.cpp

Found by PVS-Studio - https://pvs-studio.com/en/blog/posts/cpp/1003/, N18.
The value of the 'sourceDim' index is checked after it was used.

Reviewed By: zero9178

Differential Revision: https://reviews.llvm.org/D142312