select(fcmp nnan one, ...) optimization does not trigger when RHS is 0

I observed that in LLVM, the pattern:

define float @src(float %x) {
  %fcmp = fcmp nnan one float %x, 6.0
  %fsub = fsub float %x, 2.0
  %sel = select i1 %fcmp, float 2.0, float %fsub
  ret float %sel
}

can be optimized to:

define float @tgt(float %x) {
  %fcmp = fcmp nnan one float %x, 6.000000e+00
  %sel = select i1 %fcmp, float 2.000000e+00, float 4.000000e+00
  ret float %sel
}

However, when the right-hand side of the fcmp is 0, this optimization does not happen

define float @src(float %x) {
  %fcmp = fcmp nnan one float %x, 0.0
  %fsub = fsub float %x, 2.0
  %sel = select i1 %fcmp, float 2.0, float %fsub
  ret float %sel
}

This src will not be optimized to the following tgt

define float @tgt(float %x) {
  %fcmp = fcmp nnan one float %x, 0.000000e+00
  %sel = select i1 %fcmp, float 2.000000e+00, float -2.000000e+00
  ret float %sel
}

Start by running the provided @src LLVM IR through the relevant optimization pipeline and compare it with the @tgt form shown for the RHS value 6.0. Trace the fcmp/select optimization and verify that the same expected constant-folding result is produced when the comparison RHS is 0.0.