[X86] Add peephole for masked rotate amount

Extend what's currently done for shift because the HW performs this masking
implicitly:

   (rotl:i32 x, (and y, 31)) -> (rotl:i32 x, y)

I use the newly factored out multiclass that was only supporting shifts so
far.

For testing I extended my testcase for the new rotation idiom.

<rdar://problem/15295856>

llvm-svn: 203718