[MS ABI] Some code cleanups

Don't create unnecessary truncations if the result will not be used.
Also prefer preforming math before the truncation, it makes it a little
easier to reason about.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@274984 91177308-0d34-0410-b5e6-96231b3b80d8