[InstCombine] narrow width of rotate patterns, part 3

This is a longer variant for the pattern handled in
rL346713 
This one includes zexts. 

Eventually, we should canonicalize all rotate patterns 
to the funnel shift intrinsics, but we need a bit more
infrastructure to make sure the vectorizers handle those
intrinsics as well as the shift+logic ops.

https://rise4fun.com/Alive/FMn

Name: narrow rotateright
  %neg = sub i8 0, %shamt
  %rshamt = and i8 %shamt, 7
  %rshamtconv = zext i8 %rshamt to i32
  %lshamt = and i8 %neg, 7
  %lshamtconv = zext i8 %lshamt to i32
  %conv = zext i8 %x to i32
  %shr = lshr i32 %conv, %rshamtconv
  %shl = shl i32 %conv, %lshamtconv
  %or = or i32 %shl, %shr
  %r = trunc i32 %or to i8
  =>
  %maskedShAmt2 = and i8 %shamt, 7
  %negShAmt2 = sub i8 0, %shamt
  %maskedNegShAmt2 = and i8 %negShAmt2, 7
  %shl2 = lshr i8 %x, %maskedShAmt2
  %shr2 = shl i8 %x, %maskedNegShAmt2
  %r = or i8 %shl2, %shr2
llvm-svn: 346716