[AArch64] Recognize one extra br idiom

Summary:
We do not support optimizing functions with jump tables in
AArch64, but we do need to detect them. This idiom is slightly different
from the ones we've seen before. It encode jump table entries as
relative to the jump table itself instead of relative to the indirect
branch (BR) instruction.

(cherry picked from FBD18191100)

bolt/src/Target/AArch64/AArch64MCPlusBuilder.cpp

@@ -480,6 +480,22 @@ public:
       //   BR     x2
       return false;
     }
+    if (DefAdd->getOpcode() == AArch64::ADDXrs) {
+      // Covers the less common pattern where JT entries are relative to
+      // the JT itself (like x86). Seems less efficient since we can't
+      // assume the JT is aligned at 4B boundary and thus drop 2 bits from
+      // JT values.
+      // cde264:
+      //    adrp    x12, #21544960  ; 216a000
+      //    add     x12, x12, #1696 ; 216a6a0  (JT object in .rodata)
+      //    ldrsw   x8, [x12, x8, lsl #2]   --> loads e.g. 0xfeb73bd8
+      //  * add     x8, x8, x12   --> = cde278, next block
+      //    br      x8
+      // cde278:
+      //
+      // Parsed as ADDXrs reg:x8 reg:x8 reg:x12 imm:0
+      return false;
+    }
     assert(DefAdd->getOpcode() == AArch64::ADDXrx &&
            "Failed to match indirect branch!");