Silence potential overflow warning (#83272)

Cast Offset variable to int64_t type directly inside a multiplication
and function call to utilize 64-bit arithmetic.

Ensure that the multiplication will not overflow.

A static analyzer warned about this since the function expects a 64-bit
argument, but the multiplication is evaluated inside a 32-bit context.

Author: Martin Wehking

llvm/lib/Target/AMDGPU/SIRegisterInfo.cpp

@@ -1753,12 +1753,12 @@ void SIRegisterInfo::buildVGPRSpillLoadStore(SGPRSpillBuilder &SB, int Index,
     unsigned Opc = ST.enableFlatScratch() ? AMDGPU::SCRATCH_LOAD_DWORD_SADDR
                                           : AMDGPU::BUFFER_LOAD_DWORD_OFFSET;
     buildSpillLoadStore(*SB.MBB, SB.MI, SB.DL, Opc, Index, SB.TmpVGPR, false,
-                        FrameReg, Offset * SB.EltSize, MMO, SB.RS);
+                        FrameReg, (int64_t)Offset * SB.EltSize, MMO, SB.RS);
   } else {
     unsigned Opc = ST.enableFlatScratch() ? AMDGPU::SCRATCH_STORE_DWORD_SADDR
                                           : AMDGPU::BUFFER_STORE_DWORD_OFFSET;
     buildSpillLoadStore(*SB.MBB, SB.MI, SB.DL, Opc, Index, SB.TmpVGPR, IsKill,
-                        FrameReg, Offset * SB.EltSize, MMO, SB.RS);
+                        FrameReg, (int64_t)Offset * SB.EltSize, MMO, SB.RS);
     // This only ever adds one VGPR spill
     SB.MFI.addToSpilledVGPRs(1);
   }