[InstCombine] canonicalize rotate patterns with cmp/select

The cmp+branch variant of this pattern is shown in:
https://bugs.llvm.org/show_bug.cgi?id=34924
...and as discussed there, we probably can't transform
that without a rotate intrinsic. We do have that now
via funnel shift, but we're not quite ready to 
canonicalize IR to that form yet. The case with 'select'
should already be transformed though, so that's this patch.

The sequence with negation followed by masking is what we
use in the backend and partly in clang (though that part 
should be updated).

https://rise4fun.com/Alive/TplC
  %cmp = icmp eq i32 %shamt, 0
  %sub = sub i32 32, %shamt
  %shr = lshr i32 %x, %shamt
  %shl = shl i32 %x, %sub
  %or = or i32 %shr, %shl
  %r = select i1 %cmp, i32 %x, i32 %or
  =>
  %neg = sub i32 0, %shamt
  %masked = and i32 %shamt, 31
  %maskedneg = and i32 %neg, 31
  %shl2 = lshr i32 %x, %masked
  %shr2 = shl i32 %x, %maskedneg
  %r = or i32 %shl2, %shr2

llvm-svn: 346807