R600/SI: Wrap local memory pointer in AssertZExt on SI

R600/SI: Wrap local memory pointer in AssertZExt on SI

These pointers are really just offsets and they will always be
less than 16-bits.  Using AssertZExt allows us to use computeKnownBits
to prove that these values are positive.  We will use this information
in a later commit.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216277 91177308-0d34-0410-b5e6-96231b3b80d8