[DAGCombiner] widen zext of popcount based on target support

zext (ctpop X) --> ctpop (zext X)

This is a prerequisite step for canonicalizing in the other direction (narrow the popcount) in IR - PR43688:
https://bugs.llvm.org/show_bug.cgi?id=43688

I'm not sure if any other targets are affected, but I found a missing fold for PPC, so added tests based on that.
The reason we widen all the way to 64-bit in these tests is because the initial DAG looks something like this:

  t5: i8 = ctpop t4
  t6: i32 = zero_extend t5  <-- created based on IR, but unused node?
    t7: i64 = zero_extend t5

Differential Revision: https://reviews.llvm.org/D69127

Author: rotateright

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

@@ -9921,6 +9921,18 @@ SDValue DAGCombiner::visitZERO_EXTEND(SDNode *N) {
   if (SDValue NewVSel = matchVSelectOpSizesWithSetCC(N))
     return NewVSel;
 
+  // If the target does not support a pop-count in the narrow source type but
+  // does support it in the destination type, widen the pop-count to this type:
+  // zext (ctpop X) --> ctpop (zext X)
+  // TODO: Generalize this to handle starting from anyext.
+  if (N0.getOpcode() == ISD::CTPOP && N0.hasOneUse() &&
+      !TLI.isOperationLegalOrCustom(ISD::CTPOP, N0.getValueType()) &&
+      TLI.isOperationLegalOrCustom(ISD::CTPOP, VT)) {
+    SDLoc DL(N);
+    SDValue NewZext = DAG.getZExtOrTrunc(N0.getOperand(0), DL, VT);
+    return DAG.getNode(ISD::CTPOP, DL, VT, NewZext);
+  }
+
   return SDValue();
 }
 

llvm/test/CodeGen/PowerPC/popcnt-zext.ll

@@ -41,9 +41,8 @@ define i16 @zpop_i8_i16(i8 %x) {
 define i16 @popz_i8_i16(i8 %x) {
 ; FAST-LABEL: popz_i8_i16:
 ; FAST:       # %bb.0:
-; FAST-NEXT:    rlwinm 3, 3, 0, 24, 31
-; FAST-NEXT:    popcntw 3, 3
-; FAST-NEXT:    clrldi 3, 3, 32
+; FAST-NEXT:    clrldi 3, 3, 56
+; FAST-NEXT:    popcntd 3, 3
 ; FAST-NEXT:    blr
 ;
 ; SLOW-LABEL: popz_i8_i16:
@@ -114,9 +113,8 @@ define i32 @zpop_i8_i32(i8 %x) {
 define i32 @popz_i8_32(i8 %x) {
 ; FAST-LABEL: popz_i8_32:
 ; FAST:       # %bb.0:
-; FAST-NEXT:    rlwinm 3, 3, 0, 24, 31
-; FAST-NEXT:    popcntw 3, 3
-; FAST-NEXT:    clrldi 3, 3, 32
+; FAST-NEXT:    clrldi 3, 3, 56
+; FAST-NEXT:    popcntd 3, 3
 ; FAST-NEXT:    blr
 ;
 ; SLOW-LABEL: popz_i8_32:
@@ -187,9 +185,8 @@ define i32 @zpop_i16_i32(i16 %x) {
 define i32 @popz_i16_32(i16 %x) {
 ; FAST-LABEL: popz_i16_32:
 ; FAST:       # %bb.0:
-; FAST-NEXT:    rlwinm 3, 3, 0, 16, 31
-; FAST-NEXT:    popcntw 3, 3
-; FAST-NEXT:    clrldi 3, 3, 32
+; FAST-NEXT:    clrldi 3, 3, 48
+; FAST-NEXT:    popcntd 3, 3
 ; FAST-NEXT:    blr
 ;
 ; SLOW-LABEL: popz_i16_32: