"-ddump-asm-regalloc-stages" causes panic

Summary

Invoking GHC with -fregs-graph -ddump-asm-regalloc-stages causes a panic:

ghc: panic! (the 'impossible' happened)
  (GHC version 9.1.0.20201124:
	No match in record selector raPlatform

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

Older versions also panic, but give a different message:

ghc: panic! (the 'impossible' happened)
  (GHC version 8.8.4 for x86_64-unknown-linux):
	compiler/nativeGen/X86/RegInfo.hs:35:8-52: Non-exhaustive patterns in Just str


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

Steps to reproduce

In GHC root git directory:

ghc -fregs-graph -ddump-asm-regalloc-stages ./nofib/shootout/fannkuch-redux/Main.hs

Environment

• GHC version used: master/HEAD, also tested with 8.8.4 and 8.6.5

Optional:

• Operating System: Ubuntu Linux 20.04
• System Architecture: x86_64