[X86] Slightly improve our attempted error recovery for 64-bit -mno-sse2 in LowerCallResult to use FP1 if there are two return values.

If the return value is a struct of 2 doubles we need two return
registers.

If SSE2 is disabled we can't return in XMM registers like the ABI says.
After logging an error we attempt to recover by using FP0 instead
of an XMM register. But if the return needs two registers, we may have
already used FP0. So if the register we were supposed to copy to is
XMM1, copy to FP1 in the recovery instead.

This seems to fix the assertion/crash in PR44413.

Author: topperc

llvm/lib/Target/X86/X86ISelLowering.cpp

@@ -2993,11 +2993,17 @@ SDValue X86TargetLowering::LowerCallResult(
     if ((CopyVT == MVT::f32 || CopyVT == MVT::f64 || CopyVT == MVT::f128) &&
         ((Is64Bit || Ins[InsIndex].Flags.isInReg()) && !Subtarget.hasSSE1())) {
       errorUnsupported(DAG, dl, "SSE register return with SSE disabled");
-      VA.convertToReg(X86::FP0); // Set reg to FP0, avoid hitting asserts.
+      if (VA.getLocReg() == X86::XMM1)
+        VA.convertToReg(X86::FP1); // Set reg to FP1, avoid hitting asserts.
+      else
+        VA.convertToReg(X86::FP0); // Set reg to FP0, avoid hitting asserts.
     } else if (CopyVT == MVT::f64 &&
                (Is64Bit && !Subtarget.hasSSE2())) {
       errorUnsupported(DAG, dl, "SSE2 register return with SSE2 disabled");
-      VA.convertToReg(X86::FP0); // Set reg to FP0, avoid hitting asserts.
+      if (VA.getLocReg() == X86::XMM1)
+        VA.convertToReg(X86::FP1); // Set reg to FP1, avoid hitting asserts.
+      else
+        VA.convertToReg(X86::FP0); // Set reg to FP0, avoid hitting asserts.
     }
 
     // If we prefer to use the value in xmm registers, copy it out as f80 and