[llvm] [AMDGPU] fix up vop3p gisel errors (PR #136262)

[Matt Arsenault via llvm-commits]

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@@ -4318,60 +4318,577 @@ AMDGPUInstructionSelector::selectVOP3NoMods(MachineOperand &Root) const {
   }};
 }
 
-std::pair<Register, unsigned>
-AMDGPUInstructionSelector::selectVOP3PModsImpl(
-  Register Src, const MachineRegisterInfo &MRI, bool IsDOT) const {
+enum class SrcStatus {
+  IS_SAME,
+  IS_UPPER_HALF,
+  IS_LOWER_HALF,
+  IS_UPPER_HALF_NEG,
+  // This means current op = [op_upper, op_lower] and src = -op_lower.
+  IS_LOWER_HALF_NEG,
+  IS_HI_NEG,
+  // This means current op = [op_upper, op_lower] and src = [op_upper,
+  // -op_lower].
+  IS_LO_NEG,
+  IS_BOTH_NEG,
+  INVALID,
+  NEG_START = IS_UPPER_HALF_NEG,
+  NEG_END = IS_BOTH_NEG,
+  HALF_START = IS_UPPER_HALF,
+  HALF_END = IS_LOWER_HALF_NEG
+};
+
+static bool isTruncHalf(const MachineInstr *MI,
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arsenm wrote:

Document this 

https://github.com/llvm/llvm-project/pull/136262