[Fixed Point] Use FixedPointBuilder to codegen fixed-point IR.

This changes the methods in CGExprScalar to use
FixedPointBuilder to generate IR for fixed-point
conversions and operations.

Since FixedPointBuilder emits padded operations slightly
differently than the original code, some tests change.

Reviewed By: leonardchan

Differential Revision: https://reviews.llvm.org/D86282