[x86] Fix crashes in lowering bitcast instructions with the widening
mode.

This also runs the test in that mode which would reproduce the crash.
What I love is that *every single FIXME* in the test is addressed by
switching to widening.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212254 91177308-0d34-0410-b5e6-96231b3b80d8