[X86] Fix 32-bit immediate assertion and convert into backend error

The assertion previously did not work correctly because the operand was
being truncated to an `int` prior to comparison.

Author: wesleywiser

llvm/lib/Target/X86/X86RegisterInfo.cpp

@@ -965,11 +965,12 @@ X86RegisterInfo::eliminateFrameIndex(MachineBasicBlock::iterator II,
   }
 
   if (MI.getOperand(FIOperandNum+3).isImm()) {
-    // Offset is a 32-bit integer.
-    int Imm = (int)(MI.getOperand(FIOperandNum + 3).getImm());
-    int Offset = FIOffset + Imm;
-    assert((!Is64Bit || isInt<32>((long long)FIOffset + Imm)) &&
-           "Requesting 64-bit offset in 32-bit immediate!");
+    int64_t Imm = MI.getOperand(FIOperandNum + 3).getImm();
+    int Offset = FIOffset + (int)Imm;
+    if (!Is64Bit && !isInt<32>((int64_t)FIOffset + Imm))
+      MI.emitGenericError(
+          ("Requesting 64-bit offset in 32-bit immediate: " + MF.getName())
+              .str());
     if (Offset != 0 || !tryOptimizeLEAtoMOV(II))
       MI.getOperand(FIOperandNum + 3).ChangeToImmediate(Offset);
   } else {