[NVPTX] Implement NVPTX AliasAnalysis

NVPTXAliasAnalysis extends the default AA to take pointer address
spaces into account. The analysis assumes that pointers in different
address spaces do not alias, unless one of them is generic (flat)
address space.

The patch also implements pointsToConstantMemory (via
getModRefInfoMask) to expose semantic of the constant address space to
the optimizer as discussed in D112466.

Differential Revision: https://reviews.llvm.org/D124787