[opaque pointer types] Use the pointee type loaded from bitcode when constructing...

[opaque pointer types] Use the pointee type loaded from bitcode when constructing a LoadInst

Now (with a few carefully placed suppressions relating to general type
serialization, etc) we can round trip a simple load through bitcode and
textual IR without calling getElementType on a PointerType.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@235221 91177308-0d34-0410-b5e6-96231b3b80d8