[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.

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},