[AsmPrinter] Further restrict expressions supported in global initializers

lowerConstant() currently accepts a number of constant expressions
which have corresponding MC expressions, but which cannot be
evaluated as a relocatable expression (unless the operands are
constant, in which case we'll just fold the expression to a constant).

The motivation here is to clarify which constant expressions are
really needed for https://discourse.llvm.org/t/rfc-remove-most-constant-expressions/63179,
and in particular clarify that we do not need to support any
division expressions, which are particularly problematic.

Differential Revision: https://reviews.llvm.org/D127972