[InstCombine] Take in account the size in sext->lshr->trunc patterns.

Otherwise we end up miscompiling, transforming:

define i8 @tinky() {
  %sext = sext i1 1 to i16
  %hibit = lshr i16 %sext, 15
  %tr = trunc i16 %hibit to i8
  ret i8 %tr
}

into:

  %sext = sext i1 1 to i8
  ret i8 %sext

and the first get folded to ret i8 1, while the second gets folded
to ret i8 -1.

Eventually we should get rid of this transform entirely, but for now,
this at least fixes a know correctness bug.

Differential Revision:  https://reviews.llvm.org/D33338

llvm-svn: 303513