[InstructionSimplify] Apply sext/trunc after pointer stripping

Summary:
- As the pointer stripping could trace through `addrspacecast` now, need
  to sext/trunc the offset to ensure it has the same width as the
  pointer after stripping.

Reviewers: jdoerfert

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64768

llvm-svn: 366162