[llvm] [GlobalISel] Add G_SUB for computeNumSignBits (PR #158384)

[David Green via llvm-commits]

================
@@ -177,6 +177,51 @@ TEST_F(AArch64SelectionDAGTest, ComputeNumSignBits_VASHR) {
   EXPECT_EQ(DAG->ComputeNumSignBits(Fr2), 5u);
 }
 
+TEST_F(AArch64SelectionDAGTest, ComputeNumSignBits_SUB) {
----------------
davemgreen wrote:

This will be testing selection dag, not the new gisel code that has been added.

I don't know if we have tests like this already but if not this could be a separate patch.

https://github.com/llvm/llvm-project/pull/158384