[llvm] [SelectionDAG] Add BUILD_PAIR to computeKnownBits (PR #225073)

[Simon Pilgrim via llvm-commits]

================
@@ -150,4 +150,32 @@ TEST_F(X86SelectionDAGTest, computeKnownBits_FXOR) {
   EXPECT_FALSE(KnownHi2xF64.isNonNegative());
 }
 
+TEST_F(X86SelectionDAGTest, computeKnownBits_BUILD_PAIR) {
----------------
RKSimon wrote:

instead of a unit test you might be better off finding another target that makes use of BUILD_PAIR t create additional test coverage - big-endian as well if you can!

https://github.com/llvm/llvm-project/pull/225073