[opaque pointer types] Serialize the value type for atomic store instructions

[opaque pointer types] Serialize the value type for atomic store instructions

Without pointee types the space optimization of storing only the pointer
type and not the value type won't be viable - so add the extra type
information that would be missing.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@235475 91177308-0d34-0410-b5e6-96231b3b80d8