Print a=b as an assignment.

In assembly the expression a=b is parsed as an assignment, so it should be
printed as one.

This remove a truly horrible hack for producing a label with "a=.". It would
be used by codegen but would never be reached by the asm parser. Sorry I
missed this when it was first committed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211639 91177308-0d34-0410-b5e6-96231b3b80d8