[AMDGPU] Lower null pointers in static variable initializer

    For amdgcn target Clang generates addrspacecast to represent null pointers in private and local address spaces.

    In LLVM codegen, the static variable initializer is lowered by virtual function AsmPrinter::lowerConstant which is target generic. Since addrspacecast is target specific, AsmPrinter::lowerConst

    This patch overrides AsmPrinter::lowerConstant with AMDGPUAsmPrinter::lowerConstant, which is able to lower the target-specific addrspacecast in the null pointer representation so that -1 is co

    Differential Revision: https://reviews.llvm.org/D29284

llvm-svn: 294265