[Scalarizer] Avoid changing name of non-instructions

The "takeName" logic in ScalarizerVisitor::gather did not consider
that the value vector could refer to non-instructions, such as
global variables. This patch make sure that we avoid changing the
name of a value if it isn't an instruction.

Reviewed By: lebedev.ri

Differential Revision: https://reviews.llvm.org/D87685