[llvm] [AMDGPU][GlobalISel] Align `selectVOP3PMadMixModsImpl` with the `SelectionDAG` counterpart (PR #110168)

[Shilei Tian via llvm-commits]

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@@ -5312,26 +5312,20 @@ AMDGPUInstructionSelector::selectVOP3PMadMixModsImpl(MachineOperand &Root,
     // Only change Src if src modifier could be gained. In such cases new Src
     // could be sgpr but this does not violate constant bus restriction for
     // instruction that is being selected.
-    // Note: Src is not changed when there is only a simple sgpr to vgpr copy
-    // since this could violate constant bus restriction.
-    Register PeekSrc = stripCopy(Src, *MRI);
+    Src = stripBitCast(Src, *MRI);
 
     const auto CheckAbsNeg = [&]() {
       // Be careful about folding modifiers if we already have an abs. fneg is
       // applied last, so we don't want to apply an earlier fneg.
       if ((Mods & SISrcMods::ABS) == 0) {
         unsigned ModsTmp;
-        std::tie(PeekSrc, ModsTmp) = selectVOP3ModsImpl(PeekSrc);
+        std::tie(Src, ModsTmp) = selectVOP3ModsImpl(Src);
 
-        if ((ModsTmp & SISrcMods::NEG) != 0) {
+        if ((ModsTmp & SISrcMods::NEG) != 0)
           Mods ^= SISrcMods::NEG;
-          Src = PeekSrc;
-        }
 
-        if ((ModsTmp & SISrcMods::ABS) != 0) {
+        if ((ModsTmp & SISrcMods::ABS) != 0)
           Mods |= SISrcMods::ABS;
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shiltian wrote:

Not really. That is only the case for `NEG`.

https://github.com/llvm/llvm-project/pull/110168