NCG: shortcutBranch doesn't handle loops properly

The following Cmm code causes GHC to go into an infinite loop when -O and -fasm are on.

foo {
  L: goto L;
}

I do not know of any code that depends on this yet. I just tripped over it when writting tests for the CPS pass.

Trac metadata

Trac field	Value
Version	6.7
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (NCG)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system	Multiple
Architecture