[InstCombine] narrow rotate left/right patterns to eliminate zext/trunc (PR34046)

I couldn't find any smaller folds to help the cases in:
https://bugs.llvm.org/show_bug.cgi?id=34046
after:
rL310141

The truncated rotate-by-variable patterns elude all of the existing transforms because 
of multiple uses and knowledge about demanded bits and knownbits that doesn't exist 
without the whole pattern. So we need an unfortunately large pattern match. But by 
simplifying this pattern in IR, the backend is already able to generate 
rolb/rolw/rorb/rorw for x86 using its existing rotate matching logic (although
there is a likely extraneous 'and' of the rotate amount). 

Note that rotate-by-constant doesn't have this problem - smaller folds should already 
produce the narrow IR ops.

Differential Revision: https://reviews.llvm.org/D36395

llvm-svn: 310509