GlobalISel: Enable CSE of G_SELECT

[arsenm]

Summary

Stop trying to delete a select in one combine since it would
be deleting the CSE'd instruction if that happened.

[paquette]

:GTM

[arsenm]

0dc4bdd8887758d6d55337f366c66f41e42a7b97

llvm/lib/CodeGen/GlobalISel/CSEInfo.cpp

bool CSEConfigFull::shouldCSEOpc(unsigned Opc) {
   case TargetOpcode::G_IMPLICIT_DEF:
   case TargetOpcode::G_ZEXT:
   case TargetOpcode::G_SEXT:
   case TargetOpcode::G_ANYEXT:
   case TargetOpcode::G_UNMERGE_VALUES:
   case TargetOpcode::G_TRUNC:
   case TargetOpcode::G_PTR_ADD:
   case TargetOpcode::G_EXTRACT:
+  case TargetOpcode::G_SELECT:
     return true;
   }
   return false;
 }
 
 bool CSEConfigConstantOnly::shouldCSEOpc(unsigned Opc) {
   return Opc == TargetOpcode::G_CONSTANT || Opc == TargetOpcode::G_FCONSTANT ||
          Opc == TargetOpcode::G_IMPLICIT_DEF;

llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp

FoldFalse =
         Builder.buildInstr(BinOpcode, {Ty}, {SelectFalse, RHS}).getReg(0);
   } else {
     FoldTrue = Builder.buildInstr(BinOpcode, {Ty}, {LHS, SelectTrue}).getReg(0);
     FoldFalse =
         Builder.buildInstr(BinOpcode, {Ty}, {LHS, SelectFalse}).getReg(0);
   }
 
   Builder.buildSelect(Dst, SelectCond, FoldTrue, FoldFalse, MI.getFlags());
-  Observer.erasingInstr(*Select);
-  Select->eraseFromParent();
   MI.eraseFromParent();
 
   return true;
 }
 
 Optional<SmallVector<Register, 8>>
 CombinerHelper::findCandidatesForLoadOrCombine(const MachineInstr *Root) const {
   assert(Root->getOpcode() == TargetOpcode::G_OR && "Expected G_OR only!");

llvm/test/CodeGen/AMDGPU/GlobalISel/combine-fold-binop-into-select.mir

bb.0:
     ; CHECK-LABEL: name: fold_smin_into_select_s32_0
     ; CHECK: liveins: $vgpr0
     ; CHECK-NEXT: {{  $}}
     ; CHECK-NEXT: %reg:_(s32) = COPY $vgpr0
     ; CHECK-NEXT: %zero:_(s32) = G_CONSTANT i32 0
     ; CHECK-NEXT: %cond:_(s1) = G_ICMP intpred(eq), %reg(s32), %zero
     ; CHECK-NEXT: %ten:_(s32) = G_CONSTANT i32 10
     ; CHECK-NEXT: %twenty:_(s32) = G_CONSTANT i32 20
-    ; CHECK-NEXT: %smin:_(s32) = G_SELECT %cond(s1), %ten, %twenty
-    ; CHECK-NEXT: S_ENDPGM 0, implicit %smin(s32)
+    ; CHECK-NEXT: %select:_(s32) = G_SELECT %cond(s1), %ten, %twenty
+    ; CHECK-NEXT: S_ENDPGM 0, implicit %select(s32)
     %reg:_(s32) = COPY $vgpr0
     %zero:_(s32) = G_CONSTANT i32 0
     %cond:_(s1) = G_ICMP intpred(eq), %reg, %zero
     %ten:_(s32) = G_CONSTANT i32 10
     %twenty:_(s32) = G_CONSTANT i32 20
     %select:_(s32) = G_SELECT %cond, %ten, %twenty
     %thirty:_(s32) = G_CONSTANT i32 30
     %smin:_(s32) = G_SMIN %select, %thirty
bb.0:
     ; CHECK-LABEL: name: fold_umin_into_select_s32_0
     ; CHECK: liveins: $vgpr0
     ; CHECK-NEXT: {{  $}}
     ; CHECK-NEXT: %reg:_(s32) = COPY $vgpr0
     ; CHECK-NEXT: %zero:_(s32) = G_CONSTANT i32 0
     ; CHECK-NEXT: %cond:_(s1) = G_ICMP intpred(eq), %reg(s32), %zero
     ; CHECK-NEXT: %ten:_(s32) = G_CONSTANT i32 10
     ; CHECK-NEXT: %twenty:_(s32) = G_CONSTANT i32 20
-    ; CHECK-NEXT: %umin:_(s32) = G_SELECT %cond(s1), %ten, %twenty
-    ; CHECK-NEXT: S_ENDPGM 0, implicit %umin(s32)
+    ; CHECK-NEXT: %select:_(s32) = G_SELECT %cond(s1), %ten, %twenty
+    ; CHECK-NEXT: S_ENDPGM 0, implicit %select(s32)
     %reg:_(s32) = COPY $vgpr0
     %zero:_(s32) = G_CONSTANT i32 0
     %cond:_(s1) = G_ICMP intpred(eq), %reg, %zero
     %ten:_(s32) = G_CONSTANT i32 10
     %twenty:_(s32) = G_CONSTANT i32 20
     %select:_(s32) = G_SELECT %cond, %ten, %twenty
     %thirty:_(s32) = G_CONSTANT i32 30
     %umin:_(s32) = G_UMIN %select, %thirty