[SLP] Allow sequential fadd reductions

[RKSimon]

Bugzilla Link	51246
Version	trunk
OS	Windows NT
CC	@alexey-bataev,@DMG862,@sdesmalen-arm,@rotateright

Extended Description

define double @dot4f64(double* %ptrx, double* %ptry) {
  %ptrx1 = getelementptr inbounds double, double* %ptrx, i64 1
  %ptry1 = getelementptr inbounds double, double* %ptry, i64 1
  %ptrx2 = getelementptr inbounds double, double* %ptrx, i64 2
  %ptry2 = getelementptr inbounds double, double* %ptry, i64 2
  %ptrx3 = getelementptr inbounds double, double* %ptrx, i64 3
  %ptry3 = getelementptr inbounds double, double* %ptry, i64 3
  %x0 = load double, double* %ptrx, align 8
  %y0 = load double, double* %ptry, align 8
  %x1 = load double, double* %ptrx1, align 8
  %y1 = load double, double* %ptry1, align 8
  %x2 = load double, double* %ptrx2, align 8
  %y2 = load double, double* %ptry2, align 8
  %x3 = load double, double* %ptrx3, align 8
  %y3 = load double, double* %ptry3, align 8
  %mul0 = fmul double %x0, %y0
  %mul1 = fmul double %x1, %y1
  %mul2 = fmul double %x2, %y2
  %mul3 = fmul double %x3, %y3
  %dot01 = fadd double %mul0, %mul1
  %dot012 = fadd double %dot01, %mul2
  %dot0123 = fadd double %dot012, %mul3
  ret double %dot0123
}

At least on AVX targets, this (non-fast) dot product would still benefit from vectorizing the multiplication and then performing a sequential/unordered fadd reduction - but SLP currently limits reductions to associative ops.