[InstCombine] fix tests in fneg.ll

[Chenbing.Zheng]

Summary

Add flag 'nsz' in select, so that it can be fold to fabs.

In D101727, it fix Incorrect transformation of fabs, so I think
'nsz' should be added in select.

[spatel]

The TODO comment appears to be correct:
https://alive2.llvm.org/ce/z/Vwtz9q

So we should leave this test the same, but add another test that has 'nsz' on the select only?
https://alive2.llvm.org/ce/z/r_fPEu

If that sounds good, please commit an "NFC" patch to add one more test (no review on Phabricator is needed to add a test unless there are still open questions).

[Chenbing.Zheng]

In D123588#3445310, @spatel wrote:

The TODO comment appears to be correct:
https://alive2.llvm.org/ce/z/Vwtz9q

So we should leave this test the same, but add another test that has 'nsz' on the select only?
https://alive2.llvm.org/ce/z/r_fPEu

If that sounds good, please commit an "NFC" patch to add one more test (no review on Phabricator is needed to add a test unless there are still open questions).

Thanks for your explanation. I agree with you.

llvm/test/Transforms/InstCombine/fneg.ll

 ;
   %fneg = fneg float %a
   %cmp = fcmp ogt float %a, %fneg
   %sel = select i1 %cmp, float %fneg, float %a
   %fneg1 = fneg nnan ninf nsz float %sel
   ret float %fneg1
 }
 
-; TODO: This should reduce to fneg-of-fabs.
-
 define float @fnabs(float %a) {
 ; CHECK-LABEL: @fnabs(
-; CHECK-NEXT:    [[CMP:%.*]] = fcmp olt float [[A:%.*]], 0.000000e+00
-; CHECK-NEXT:    [[A_NEG:%.*]] = fneg fast float [[A]]
-; CHECK-NEXT:    [[FNEG1:%.*]] = select fast i1 [[CMP]], float [[A]], float [[A_NEG]]
+; CHECK-NEXT:    [[TMP1:%.*]] = call nsz float @llvm.fabs.f32(float [[A:%.*]])
+; CHECK-NEXT:    [[FNEG1:%.*]] = fneg fast float [[TMP1]]
 ; CHECK-NEXT:    ret float [[FNEG1]]
 ;
   %fneg = fneg float %a
   %cmp = fcmp olt float %a, %fneg
-  %sel = select i1 %cmp, float %fneg, float %a
+  %sel = select nsz i1 %cmp, float %fneg, float %a
   %fneg1 = fneg fast float %sel
   ret float %fneg1
 }
 
 define float @select_fneg_both(float %x, float %y, i1 %b) {
 ; CHECK-LABEL: @select_fneg_both(
 ; CHECK-NEXT:    [[S_V:%.*]] = select i1 [[B:%.*]], float [[X:%.*]], float [[Y:%.*]]
 ; CHECK-NEXT:    ret float [[S_V]]