[InstCombine] allow more fast-math-flags to propagate in fneg-of-select fold

We were conservatively intersecting flags, but we can take the union here
because forbidden special values (nsz/nnan/ninf) are not altered by fneg.
So if they were guaranteed not present on the select or fneg, then they
are guaranteed not present on the new select. Alive2 appears to agree on
the test diffs (reduced to not include irrelevant flags like reassoc):
https://alive2.llvm.org/ce/z/ViqqrO

This prevents a potential regression if we tighten up the FMF behavior
for fabs with NAN as suggested in issue #59279.

Author: rotateright

llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp

@@ -2408,14 +2408,18 @@ Instruction *InstCombinerImpl::visitFNeg(UnaryOperator &I) {
   Value *Cond;
   if (match(Op, m_OneUse(m_Select(m_Value(Cond), m_Value(X), m_Value(Y))))) {
     // Unlike most transforms, this one is not safe to propagate nsz unless
-    // it is present on the original select. (We are conservatively intersecting
-    // the nsz flags from the select and root fneg instruction.)
+    // it is present on the original select. We union the flags from the select
+    // and fneg and then remove nsz if needed.
     auto propagateSelectFMF = [&](SelectInst *S, bool CommonOperand) {
       S->copyFastMathFlags(&I);
-      if (auto *OldSel = dyn_cast<SelectInst>(Op))
+      if (auto *OldSel = dyn_cast<SelectInst>(Op)) {
+        FastMathFlags FMF = I.getFastMathFlags();
+        FMF |= OldSel->getFastMathFlags();
+        S->setFastMathFlags(FMF);
         if (!OldSel->hasNoSignedZeros() && !CommonOperand &&
             !isGuaranteedNotToBeUndefOrPoison(OldSel->getCondition()))
           S->setHasNoSignedZeros(false);
+      }
     };
     // -(Cond ? -P : Y) --> Cond ? P : -Y
     Value *P;

llvm/test/Transforms/InstCombine/fneg.ll

@@ -629,7 +629,7 @@ define float @select_fneg_true(float %x, float %y, i1 %b) {
 define <2 x float> @select_fneg_false(<2 x float> %x, <2 x float> %y, <2 x i1> %b) {
 ; CHECK-LABEL: @select_fneg_false(
 ; CHECK-NEXT:    [[X_NEG:%.*]] = fneg nnan nsz <2 x float> [[X:%.*]]
-; CHECK-NEXT:    [[R:%.*]] = select nnan <2 x i1> [[B:%.*]], <2 x float> [[X_NEG]], <2 x float> [[Y:%.*]]
+; CHECK-NEXT:    [[R:%.*]] = select nnan ninf <2 x i1> [[B:%.*]], <2 x float> [[X_NEG]], <2 x float> [[Y:%.*]]
 ; CHECK-NEXT:    ret <2 x float> [[R]]
 ;
   %ny = fneg nnan <2 x float> %y
@@ -772,7 +772,7 @@ define float @select_fneg_use1(float %x, float %y, i1 %b) {
 ; CHECK-NEXT:    [[NX:%.*]] = fneg ninf float [[X:%.*]]
 ; CHECK-NEXT:    call void @use(float [[NX]])
 ; CHECK-NEXT:    [[Y_NEG:%.*]] = fneg float [[Y:%.*]]
-; CHECK-NEXT:    [[R:%.*]] = select i1 [[B:%.*]], float [[X]], float [[Y_NEG]]
+; CHECK-NEXT:    [[R:%.*]] = select fast i1 [[B:%.*]], float [[X]], float [[Y_NEG]]
 ; CHECK-NEXT:    ret float [[R]]
 ;
   %nx = fneg ninf float %x