machinst/x64: teach regalloc what FP instructions are moves;

and cosmetic changes after #1665 landed.

cranelift/codegen/src/isa/x64/abi.rs

@@ -315,9 +315,7 @@ impl ABIBody for X64ABIBody {
                         from_reg.to_reg(),
                         Writable::<Reg>::from_reg(to_reg.to_reg()),
                     ))
-                } else if to_reg.get_class() == RegClass::V128
-                    || to_reg.get_class() == RegClass::V128
-                {
+                } else if to_reg.get_class() == RegClass::V128 {
                     ret.push(Inst::xmm_r_r(
                         SSE_Op::SSE2_Movsd,
                         from_reg.to_reg(),

cranelift/codegen/src/isa/x64/inst/mod.rs

@@ -811,6 +811,11 @@ impl MachInst for Inst {
         // %reg.
         match self {
             Self::Mov_R_R { is_64, src, dst } if *is_64 => Some((*dst, *src)),
+            Self::XMM_R_R { op, src, dst }
+                if *op == SSE_Op::SSE_Movss || *op == SSE_Op::SSE2_Movsd =>
+            {
+                Some((*dst, *src))
+            }
             _ => None,
         }
     }