fix infinite loop in instcombine with more than 4GB memcpy

fix infinite loop in instcombine with more than 4GB memcpy

- memcpy size is wrongly truncated into 32-bit and treat 8GB memcpy is
  0-sized memcpy
- as 0-sized memcpy/memset is already removed before SimplifyMemTransfer
  and SimplifyMemSet in visitCallInst, replace 0 checking with
  assertions.
- replace getZExtValue() with getLimitedValue() according to
  Eli Friedman

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@161923 91177308-0d34-0410-b5e6-96231b3b80d8