[ValueTracking] Compute KnownFP state from recursive select/phi.

Given a recursive phi with select:
 %p = phi [ 0, entry ], [ %sel, loop]
 %sel = select %c, %other, %p

The fp state can be calculated using the knowledge that the select/phi pair can
only be the initial state (0 here) or from %other. This adds a short-cut into
computeKnownFPClass for PHI to detect that the select is recursive back to the
phi, and if so use the state from the other operand.

This helps to address a regression from #83200.

Author: davemgreen

llvm/lib/Analysis/ValueTracking.cpp

@@ -5999,6 +5999,13 @@ void computeKnownFPClass(const Value *V, const APInt &DemandedElts,
         if (IncValue == P)
           continue;
 
+        // If the Use is a select of this phi, use the fp class of the other
+        // operand to break the recursion.
+        Value *V;
+        if (match(IncValue, m_Select(m_Value(), m_Specific(P), m_Value(V))) ||
+            match(IncValue, m_Select(m_Value(), m_Value(V), m_Specific(P))))
+          IncValue = V;
+
         KnownFPClass KnownSrc;
         // Recurse, but cap the recursion to two levels, because we don't want
         // to waste time spinning around in loops. We need at least depth 2 to

llvm/test/Transforms/LoopVectorize/ARM/mve-selectandorcost.ll

@@ -45,8 +45,8 @@ define float @test(ptr nocapture readonly %pA, ptr nocapture readonly %pB, i32 %
 ; CHECK-NEXT:    [[TMP7:%.*]] = fsub fast <4 x float> [[WIDE_LOAD]], [[WIDE_LOAD7]]
 ; CHECK-NEXT:    [[TMP8:%.*]] = call fast <4 x float> @llvm.fabs.v4f32(<4 x float> [[TMP7]])
 ; CHECK-NEXT:    [[TMP9:%.*]] = fdiv fast <4 x float> [[TMP8]], [[TMP6]]
-; CHECK-NEXT:    [[TMP10:%.*]] = fadd fast <4 x float> [[TMP9]], [[VEC_PHI]]
-; CHECK-NEXT:    [[PREDPHI]] = select <4 x i1> [[DOTNOT9]], <4 x float> [[VEC_PHI]], <4 x float> [[TMP10]]
+; CHECK-NEXT:    [[TMP10:%.*]] = select <4 x i1> [[DOTNOT9]], <4 x float> <float -0.000000e+00, float -0.000000e+00, float -0.000000e+00, float -0.000000e+00>, <4 x float> [[TMP9]]
+; CHECK-NEXT:    [[PREDPHI]] = fadd fast <4 x float> [[VEC_PHI]], [[TMP10]]
 ; CHECK-NEXT:    [[INDEX_NEXT]] = add nuw i32 [[INDEX]], 4
 ; CHECK-NEXT:    [[TMP11:%.*]] = icmp eq i32 [[INDEX_NEXT]], [[N_VEC]]
 ; CHECK-NEXT:    br i1 [[TMP11]], label [[MIDDLE_BLOCK:%.*]], label [[VECTOR_BODY]], !llvm.loop [[LOOP0:![0-9]+]]