PowerPC: Panic in linear register allocator

Job #698218 failed for abcc7a04:

ghc-stage1: panic! (the 'impossible' happened)
  GHC version 9.3.20210606:
	RegAllocLinear.getFreeRegs: Bad register class
  F
  Call stack:
      CallStack (from HasCallStack):
        callStackDoc, called at compiler/GHC/Utils/Panic.hs:186:37 in ghc:GHC.Utils.Panic
        pprPanic, called at compiler/GHC/CmmToAsm/Reg/Linear/PPC.hs:51:19 in ghc:GHC.CmmToAsm.Reg.Linear.PPC
Please report this as a GHC bug:  https://www.haskell.org/ghc/reportabug
make[1]: *** [rts/ghc.mk:322: rts/dist/build/StgStartup.o] Error 1
make[1]: *** Waiting for unfinished jobs....
ghc-stage1: panic! (the 'impossible' happened)
  GHC version 9.3.20210606:
	RegAllocLinear.getFreeRegs: Bad register class
  F
  Call stack:
      CallStack (from HasCallStack):
        callStackDoc, called at compiler/GHC/Utils/Panic.hs:186:37 in ghc:GHC.Utils.Panic
        pprPanic, called at compiler/GHC/CmmToAsm/Reg/Linear/PPC.hs:51:19 in ghc:GHC.CmmToAsm.Reg.Linear.PPC
Please report this as a GHC bug:  https://www.haskell.org/ghc/reportabug
make[1]: *** [rts/ghc.mk:322: rts/dist/build/Updates.o] Error 1
ghc-stage1: panic! (the 'impossible' happened)
  GHC version 9.3.20210606:
	RegAllocLinear.getFreeRegs: Bad register class
  F
  Call stack:
      CallStack (from HasCallStack):
        callStackDoc, called at compiler/GHC/Utils/Panic.hs:186:37 in ghc:GHC.Utils.Panic
        pprPanic, called at compiler/GHC/CmmToAsm/Reg/Linear/PPC.hs:51:19 in ghc:GHC.CmmToAsm.Reg.Linear.PPC
Please report this as a GHC bug:  https://www.haskell.org/ghc/reportabug
make[1]: *** [rts/ghc.mk:322: rts/dist/build/StgStdThunks.o] Error 1

This regression was introduced after 737b0ae1.