The DAGCombiner was threading select over pairs of extending loads even

The DAGCombiner was threading select over pairs of extending loads even
if the extension types were not the same.  The result was that if you
fed a select with sext and zext loads, as in the testcase, then it
would get turned into a zext (or sext) of the select, which is wrong
in the cases when it should have been an sext (resp. zext).  Reported
and diagnosed by Sebastien Deldon.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@119728 91177308-0d34-0410-b5e6-96231b3b80d8