[AArch64][GlobalISel] Legalize more CTPOP vector types.

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.

Author: davemgreen

llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp

@@ -6140,6 +6140,7 @@ LegalizerHelper::moreElementsVector(MachineInstr &MI, unsigned TypeIdx,
   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);

llvm/lib/Target/AArch64/GISel/AArch64LegalizerInfo.cpp

@@ -323,7 +323,13 @@ AArch64LegalizerInfo::AArch64LegalizerInfo(const AArch64Subtarget &ST)
       .clampScalar(0, s32, s128)
       .widenScalarToNextPow2(0)
       .minScalarEltSameAsIf(always, 1, 0)
-      .maxScalarEltSameAsIf(always, 1, 0);
+      .maxScalarEltSameAsIf(always, 1, 0)
+      .clampNumElements(0, v8s8, v16s8)
+      .clampNumElements(0, v4s16, v8s16)
+      .clampNumElements(0, v2s32, v4s32)
+      .clampNumElements(0, v2s64, v2s64)
+      .moreElementsToNextPow2(0)
+      .scalarizeIf(scalarOrEltWiderThan(0, 64), 0);
 
   getActionDefinitionsBuilder(G_CTLZ)
       .legalFor({{s32, s32},