[ARM GlobalISel] Move the check for Thumb higher up

[ARM GlobalISel] Move the check for Thumb higher up

We're currently bailing out for Thumb targets while lowering formal
parameters, but there used to be some other checks before it, which
could've caused some functions (e.g. those without formal parameters) to
sneak through unnoticed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@317312 91177308-0d34-0410-b5e6-96231b3b80d8