[Fixed Point] Move the compassign LHS type correction a bit further down. NFCI.

Summary:
We can simplify the LHSTy correction for
fixed-point compassign by moving it below
the point where we know we have a compound
assignment.

Also, we shouldn't look at the LHS and RHS
separately; look at the computation result
type instead.

Looking at the LHS and RHS is also wrong
for compassigns with fixed and floating
point (though this does not work upstream
yet).

Reviewers: leonardchan

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D78294