Fix doc of `llvm.bitreverse.iN`

Summary:
The return type is `iN` rather than always `i16`

Seems to be a typo in https://reviews.llvm.org/rL252878 .

Reviewers: jmolloy

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D27047

llvm-svn: 287769