Fix a miscompile in the DAG combiner. Previously, we would incorrectly

try to reduce the width of this load, and would end up transforming:

  (truncate (lshr (sextload i48 <ptr> as i64), 32) to i32)
to
  (truncate (zextload i32 <ptr+4> as i64) to i32)

We lost the sext attached to the load while building the narrower i32
load, and replaced it with a zext because lshr always zext's the
results. Instead, bail out of this combine when there is a conflict
between a sextload and a zext narrowing. The rest of the DAG combiner
still optimize the code down to the proper single instruction:

  movswl 6(...),%eax

Which is exactly what we wanted. Previously we read past the end *and*
missed the sign extension:

  movl 6(...), %eax

llvm-svn: 169802