[LLParser] Simplify type checking in ParseArithmetic and ParseUnaryOp.

Summary:
These methods previously took a 0, 1, or 2 to indicate what types were allowed, but the 0 encoding which meant both fp and integer types has been unused for years. Its leftover from when add/sub/mul used to be shared between int and fp

Simplify it by changing it to just a bool to distinquish int and fp.

Reviewers: spatel, dblaikie, jyknight, arsenm

Reviewed By: spatel

Subscribers: wdng, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D61561

llvm-svn: 359986