[ARM] PR32379 - fix CMOV operands in computeKnownBits

[arielb1]

Summary

ARMISD::CMOV has operands as the following:

CMOV <on-false> <on-true> <armcc> <ccr> <cmp>

In particular, the return value of it is either operand 0 or operand 1, so computeKnownBits can return the mutually-known bits in these operands - it used to return the mutually-known bits in operands *1* and *2*, which was bogus.

[efriedma]

https://reviews.llvm.org/D31253 has a slightly cleaner version of this fix.

[arielb1]

Ninja'd

lib/Target/ARM/ARMISelLowering.cpp

     const APInt &Mask = CI->getAPIntValue();
     KnownZero &= Mask;
     KnownOne &= Mask;
     return;
   }
   if (Op.getOpcode() == ARMISD::CMOV) {
     APInt KZ2(KnownZero.getBitWidth(), 0);
     APInt KO2(KnownOne.getBitWidth(), 0);
-    computeKnownBits(DAG, Op.getOperand(1), KnownZero, KnownOne);
-    computeKnownBits(DAG, Op.getOperand(2), KZ2, KO2);
+    computeKnownBits(DAG, Op.getOperand(0), KnownZero, KnownOne);
+    computeKnownBits(DAG, Op.getOperand(1), KZ2, KO2);
 
     KnownZero &= KZ2;
     KnownOne &= KO2;
     return;
   }
   return DAG.computeKnownBits(Op, KnownZero, KnownOne);
 }