Avoid allocating the same physreg to multiple virtregs in one instruction.

While that approach works wonders for register pressure, it tends to break
everything.

This should unbreak the arm-linux builder and fix a number of miscompilations.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@103946 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/RegAllocFast.cpp

@@ -471,6 +471,7 @@ void RAFast::allocVirtReg(MachineInstr *MI, LiveRegEntry &LRE, unsigned Hint) {
 
   unsigned BestReg = 0, BestCost = spillImpossible;
   for (TargetRegisterClass::iterator I = AOB; I != AOE; ++I) {
+    if (UsedInInstr.test(*I)) continue;
     unsigned Cost = calcSpillCost(*I);
     // Cost is 0 when all aliases are already disabled.
     if (Cost == 0)