Clean up the helpers used to compute the usual arithmetic conversions' result

Clean up the helpers used to compute the usual arithmetic conversions' result
type. Localize all of the logic within a single function rather than spreading
it throughout the class.

Also fixes a buglet where we failed to check for a RHS arithmetic type wider
than the LHS and return its canonical type. I've yet to produce a test case
that breaks because of this, but it was spotted by inspection by folks on the
IRC channel and is obviously correct now.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@121633 91177308-0d34-0410-b5e6-96231b3b80d8