panic in allocateRegsAndSpill when using FMA primops

Summary

I get the following error when compiling some code containing FMA primops.

$ ghc -mfma -O2 TwoProdFMA.hs
[1 of 1] Compiling TwoProdFMA       ( TwoProdFMA.hs, TwoProdFMA.o ) [Flags changed]

<no location info>: error:
    panic! (the 'impossible' happened)
  GHC version 9.9.20231105:
        allocateRegsAndSpill: Cannot read from uninitialized register
  %vDouble_nCE
  Call stack:
      CallStack (from HasCallStack):
        callStackDoc, called at compiler/GHC/Utils/Panic.hs:186:37 in ghc-9.9-inplace:GHC.Utils.Panic
        pprPanic, called at compiler/GHC/CmmToAsm/Reg/Linear.hs:837:20 in ghc-9.9-inplace:GHC.CmmToAsm.Reg.Linear
  CallStack (from HasCallStack):
    panic, called at compiler/GHC/Utils/Error.hs:511:29 in ghc-9.9-inplace:GHC.Utils.Error


Please report this as a GHC bug:  https://www.haskell.org/ghc/reportabug

Steps to reproduce

Compile the following program with -mfma -O2:

{-# LANGUAGE MagicHash #-}
{-# LANGUAGE UnboxedTuples #-}
module TwoProdFMA where
import GHC.Exts

twoProductFloat# :: Float# -> Float# -> (# Float#, Float# #)
twoProductFloat# x y = let !r = x `timesFloat#` y
                       in (# r, fmsubFloat# x y r #)

Expected behavior

The program should compile fine.

Environment

• GHC version used: 9.8.1, master (e451139f)
• Operating System: Linux (also confirmed on Windows with 9.8.1)
• System Architecture: x86_64 (does not reproduce on AArch64 macOS)