causing assertion failures in getSExtValue().
Fix it by making highWordBits actually contain what its name says,
and add some more unit-tests for APInt.
This fixes PR3419.

llvm-svn: 63107

The way this worked before was to test APInt by running
"lli -force-interpreter=true" knowing the lli uses APInt under the hood to
store its values. Now, we test APInt directly.

llvm-svn: 62514