[Scalarizer] Avoid updating the name of globals

The "takeName" logic at the end of ScalarizerVisitor::finish
could end up renaming global variables when having simplified
and extractelement instruction to simply pick a single vector
element. If the input vector to the extractelement instruction
held pointers to global variables we ended up renaming the global
variable.
The patch make sure we only take the name of the replaced Op when
we have added new instructions that might need a useful name.

Reviewed By: lebedev.ri

Differential Revision: https://reviews.llvm.org/D86472