Disable loop unroll pass

Ok, let‘s stop the open64 "polution".

Whether the design is as you stated doesn‘t simpler, the code before
and after the change already tells us.

We take detailed investigation on gcc support for hwloop, then we come
to the conclusion we are essentially the same. So i think the idea can
be shared among different compilers, general abstract tripcount, make
pseudo operators for indentification and special handling, that‘s what
i think might help.

Shuxin Yang <shuxin.llvm@gmail.com>编写:

Hi, Gang:

   I don't want to discuss Open64 internal in LLVM mailing list. Let us
only focus on the design per se.
As your this mail and your previous mail combined give me a impression
that :

    The only reason you introduce the specific operator for HW loop in
Scalar Opt simply because
you have hard time in figure out the trip count in CodeGen.

    This might be true for Open64's CodeGen (I don't want to discuss
this issue on this mailling list), but
in general it is not true for other compilers.

    I'm dubious about "It greatly simplify the design". The downstream
passes need to be fully aware
of this new operator, which doesn't make things any simpler.