[llvm] Replace uses of Type::getPointerTo (NFC)

[llvm] Replace uses of Type::getPointerTo (NFC)

Partial progress towards removing in-tree uses of `Type::getPointerTo`,
before we can deprecate the API.

If the API is used solely to support an unnecessary bitcast, get rid of
the bitcast as well.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D153933