[AMDGPU] Fix assertion failure on mad with negative immediate addend

Without this, the new test case would fail with:

AMDGPUInstPrinter.cpp:545: void llvm::AMDGPUInstPrinter::printImmediate64(uint64_t, const llvm::MCSubtargetInfo &, llvm::raw_ostream &): Assertion `isUInt<32>(Imm) || Imm == 0x3fc45f306dc9c882' failed.

Differential Revision: https://reviews.llvm.org/D128435