[Fixed Point Arithmetic] Refactor fixed point casts

Summary:
- Added names for some emitted values (such as "tobool" for
  the result of a cast to boolean).
- Replaced explicit IRBuilder request for doing sext/zext/trunc
  by using CreateIntCast instead.
- Simplify code for emitting satuation into one if-statement
  for clamping to max, and one if-statement for clamping to min.

Reviewers: leonardchan, ebevhan

Reviewed By: leonardchan

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D53707

llvm-svn: 345398