[SCEV] Use LLVM_MARK_AS_BITMASK_ENUM in SCEV.

Summary:
Obviates the need for mask/clear/setFlags helpers.

There are some expressions here which can be simplified, but to keep
this easy to review, I have not simplified them in this patch.

No functional change.

Reviewers: sanjoy

Subscribers: hiraditya, llvm-commits

Differential Revision: https://reviews.llvm.org/D48237

llvm-svn: 334874