[InstCombine] Negate ABS/NABS patterns by swapping the select operands to remove the negation

[craig.topper]

Summary

This is split out from D47041.

[spatel]

LGTM.

lib/Transforms/InstCombine/InstCombineAddSub.cpp

         Value *ShAmtOp = cast<Instruction>(Op1)->getOperand(1);
         return BinaryOperator::CreateLShr(X, ShAmtOp);
       }
+
+      if (Op1->hasOneUse()) {
+        Value *LHS, *RHS;
+        SelectPatternFlavor SPF = matchSelectPattern(Op1, LHS, RHS).Flavor;
+        if (SPF == SPF_ABS || SPF == SPF_NABS) {
+          // This is a negate of an ABS/NABS pattern. Just swap the operands
+          // of the select.
+          SelectInst *SI = cast<SelectInst>(Op1);
+          Value *TrueVal = SI->getTrueValue();
+          Value *FalseVal = SI->getFalseValue();
+          SI->setTrueValue(FalseVal);
+          SI->setFalseValue(TrueVal);
+          // Don't swap prof metadata, we didn't change the branch behavior.
+          return replaceInstUsesWith(I, SI);
+        }
+      }
     }
 
     // Turn this into a xor if LHS is 2^n-1 and the remaining bits are known

test/Transforms/InstCombine/abs-1.ll

   ret i8 %abs
 }
 
+define i8 @negate_abs(i8 %x) {
+; CHECK-LABEL: @negate_abs(
+; CHECK-NEXT:    [[N:%.*]] = sub i8 0, [[X:%.*]]
+; CHECK-NEXT:    [[C:%.*]] = icmp slt i8 [[X]], 0
+; CHECK-NEXT:    [[S:%.*]] = select i1 [[C]], i8 [[X]], i8 [[N]]
+; CHECK-NEXT:    ret i8 [[S]]
+;
+  %n = sub i8 0, %x
+  %c = icmp slt i8 %x, 0
+  %s = select i1 %c, i8 %n, i8 %x
+  %r = sub i8 0, %s
+  ret i8 %r
+}
+
+define <2 x i8> @negate_nabs(<2 x i8> %x) {
+; CHECK-LABEL: @negate_nabs(
+; CHECK-NEXT:    [[N:%.*]] = sub <2 x i8> zeroinitializer, [[X:%.*]]
+; CHECK-NEXT:    [[C:%.*]] = icmp slt <2 x i8> [[X]], zeroinitializer
+; CHECK-NEXT:    [[S:%.*]] = select <2 x i1> [[C]], <2 x i8> [[N]], <2 x i8> [[X]]
+; CHECK-NEXT:    ret <2 x i8> [[S]]
+;
+  %n = sub <2 x i8> zeroinitializer, %x
+  %c = icmp slt <2 x i8> %x, zeroinitializer
+  %s = select <2 x i1> %c, <2 x i8> %x, <2 x i8> %n
+  %r = sub <2 x i8> zeroinitializer, %s
+  ret <2 x i8> %r
+}