[InstCombine] Fix assertion failure in issue80597 (#80614)

The assertion in #80597 failed when we were trying to compute known bits
of a value in an unreachable BB.

https://github.com/llvm/llvm-project/blob/859b09da08c2a47026ba0a7d2f21b7dca705864d/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp#L749-L810

In this case, `SignBits` is 30 (deduced from instr info), but `Known` is
`10000101010111010011110101000?0?00000000000000000000000000000000`
(deduced from dom cond). Setting high bits of `lshr Known, 1` will lead
to conflict.

This patch masks out high bits of `Known.Zero` to address this problem.

Fixes #80597.

(cherry picked from commit cb8d83a)

llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp

@@ -802,6 +802,9 @@ Value *InstCombinerImpl::SimplifyDemandedUseBits(Value *V, APInt DemandedMask,
         return InsertNewInstWith(LShr, I->getIterator());
       } else if (Known.One[BitWidth-ShiftAmt-1]) { // New bits are known one.
         Known.One |= HighBits;
+        // SignBits may be out-of-sync with Known.countMinSignBits(). Mask out
+        // high bits of Known.Zero to avoid conflicts.
+        Known.Zero &= ~HighBits;
       }
     } else {
       computeKnownBits(I, Known, Depth, CxtI);

llvm/test/Transforms/InstCombine/pr80597.ll

@@ -0,0 +1,33 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 4
+; RUN: opt -S -passes=instcombine < %s | FileCheck %s
+
+define i64 @pr80597(i1 %cond) {
+; CHECK-LABEL: define i64 @pr80597(
+; CHECK-SAME: i1 [[COND:%.*]]) {
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[ADD:%.*]] = select i1 [[COND]], i64 0, i64 -12884901888
+; CHECK-NEXT:    [[SEXT1:%.*]] = add nsw i64 [[ADD]], 8836839514384105472
+; CHECK-NEXT:    [[CMP:%.*]] = icmp ult i64 [[SEXT1]], -34359738368
+; CHECK-NEXT:    br i1 [[CMP]], label [[IF_THEN:%.*]], label [[IF_ELSE:%.*]]
+; CHECK:       if.else:
+; CHECK-NEXT:    [[SEXT2:%.*]] = ashr exact i64 [[ADD]], 1
+; CHECK-NEXT:    [[ASHR:%.*]] = or i64 [[SEXT2]], 4418419761487020032
+; CHECK-NEXT:    ret i64 [[ASHR]]
+; CHECK:       if.then:
+; CHECK-NEXT:    ret i64 0
+;
+entry:
+  %add = select i1 %cond, i64 0, i64 4294967293
+  %add8 = shl i64 %add, 32
+  %sext1 = add i64 %add8, 8836839514384105472
+  %cmp = icmp ult i64 %sext1, -34359738368
+  br i1 %cmp, label %if.then, label %if.else
+
+if.else:
+  %sext2 = or i64 %add8, 8836839522974040064
+  %ashr = ashr i64 %sext2, 1
+  ret i64 %ashr
+
+if.then:
+  ret i64 0
+}