[GlobalOpt] Do not shrink global to bool for an unfavorable AS

Do not call `TryToShrinkGlobalToBoolean` for address spaces
that don't allow initializers. It inserts an initializer value
while shrinking to bool. Used the target hook introduced with
D109337 to skip this call for the restricted address spaces.

Reviewed By: tra

Differential Revision: https://reviews.llvm.org/D109823