Register allocation overlooks conflicts between flag registers

[vbgl]

When compiling the following program, register allocation succeeds and happily allocates the cf variable to the CF register. Compilation later crashes with the unhelpful “variable already set”.

export
fn test(reg u64 x) -> reg u64 {
  reg u64 r;
  reg bool cf;
  cf, x = x + x;
  r = #set0();
  r = x if cf;
  return r;
}

This is a regression of the glob-array3 branch, introduced by commit d316054.