[GlobalISel] Make use of KnownBits::computeForAddSub

[foad]

Summary

This is mostly NFC. computeForAddSub may give more precise results in
some cases, but that doesn't seem to affect any existing GlobalISel
tests.

[merge_guards_bot]

Unit tests: pass. 62197 tests passed, 0 failed and 815 were skipped.

clang-tidy: pass.

clang-format: pass.

Build artifacts: diff.json, clang-tidy.txt, clang-format.patch, CMakeCache.txt, console-log.txt, test-results.xml

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llvm/lib/CodeGen/GlobalISel/GISelKnownBits.cpp

case TargetOpcode::G_CONSTANT: {
     Known.Zero = ~Known.One;
     break;
   }
   case TargetOpcode::G_FRAME_INDEX: {
     computeKnownBitsForFrameIndex(R, Known, DemandedElts);
     break;
   }
   case TargetOpcode::G_SUB: {
-    // If low bits are known to be zero in both operands, then we know they are
-    // going to be 0 in the result. Both addition and complement operations
-    // preserve the low zero bits.
-    computeKnownBitsImpl(MI.getOperand(1).getReg(), Known2, DemandedElts,
+    computeKnownBitsImpl(MI.getOperand(1).getReg(), Known, DemandedElts,
                          Depth + 1);
-    unsigned KnownZeroLow = Known2.countMinTrailingZeros();
-    if (KnownZeroLow == 0)
-      break;
     computeKnownBitsImpl(MI.getOperand(2).getReg(), Known2, DemandedElts,
                          Depth + 1);
-    KnownZeroLow = std::min(KnownZeroLow, Known2.countMinTrailingZeros());
-    Known.Zero.setLowBits(KnownZeroLow);
+    Known = KnownBits::computeForAddSub(/*Add*/ false, /*NSW*/ false, Known,
+                                        Known2);
     break;
   }
   case TargetOpcode::G_XOR: {
     computeKnownBitsImpl(MI.getOperand(2).getReg(), Known, DemandedElts,
                          Depth + 1);
     computeKnownBitsImpl(MI.getOperand(1).getReg(), Known2, DemandedElts,
                          Depth + 1);
 
     // Output known-0 bits are known if clear or set in both the LHS & RHS.
     APInt KnownZeroOut = (Known.Zero & Known2.Zero) | (Known.One & Known2.One);
     // Output known-1 are known to be set if set in only one of the LHS, RHS.
     Known.One = (Known.Zero & Known2.One) | (Known.One & Known2.Zero);
     Known.Zero = KnownZeroOut;
     break;
   }
   case TargetOpcode::G_PTR_ADD: {
     // G_PTR_ADD is like G_ADD. FIXME: Is this true for all targets?
     LLT Ty = MRI.getType(MI.getOperand(1).getReg());
     if (DL.isNonIntegralAddressSpace(Ty.getAddressSpace()))
       break;
     LLVM_FALLTHROUGH;
   }
   case TargetOpcode::G_ADD: {
-    // Output known-0 bits are known if clear or set in both the low clear bits
-    // common to both LHS & RHS.  For example, 8+(X<<3) is known to have the
-    // low 3 bits clear.
-    // Output known-0 bits are also known if the top bits of each input are
-    // known to be clear. For example, if one input has the top 10 bits clear
-    // and the other has the top 8 bits clear, we know the top 7 bits of the
-    // output must be clear.
-    computeKnownBitsImpl(MI.getOperand(1).getReg(), Known2, DemandedElts,
+    computeKnownBitsImpl(MI.getOperand(1).getReg(), Known, DemandedElts,
                          Depth + 1);
-    unsigned KnownZeroHigh = Known2.countMinLeadingZeros();
-    unsigned KnownZeroLow = Known2.countMinTrailingZeros();
     computeKnownBitsImpl(MI.getOperand(2).getReg(), Known2, DemandedElts,
                          Depth + 1);
-    KnownZeroHigh = std::min(KnownZeroHigh, Known2.countMinLeadingZeros());
-    KnownZeroLow = std::min(KnownZeroLow, Known2.countMinTrailingZeros());
-    Known.Zero.setLowBits(KnownZeroLow);
-    if (KnownZeroHigh > 1)
-      Known.Zero.setHighBits(KnownZeroHigh - 1);
+    Known =
+        KnownBits::computeForAddSub(/*Add*/ true, /*NSW*/ false, Known, Known2);
     break;
   }
   case TargetOpcode::G_AND: {
     // If either the LHS or the RHS are Zero, the result is zero.
     computeKnownBitsImpl(MI.getOperand(2).getReg(), Known, DemandedElts,
                          Depth + 1);
     computeKnownBitsImpl(MI.getOperand(1).getReg(), Known2, DemandedElts,
                          Depth + 1);