[AArch64][GlobalISel] Legalize more CTPOP vector types. (#131513)

Similar to other operations, s8, s16 s32 and s64 vector elements are
clamped to legal vector sizes, odd number of elements are widened to the
next power-2 and s128 is scalarized.

This helps legalize cttz as well as ctpop.
diff --git a/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp b/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
index 04b1f84..208fa84 100644
--- a/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
+++ b/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
@@ -6140,6 +6140,7 @@
   case TargetOpcode::G_SEXT_INREG:
   case TargetOpcode::G_ABS:
   case TargetOpcode::G_CTLZ:
+  case TargetOpcode::G_CTPOP:
     if (TypeIdx != 0)
       return UnableToLegalize;
     Observer.changingInstr(MI);